The build phase is where marathon training becomes race-specific. After establishing an aerobic foundation in the base phase, runners introduce controlled intensity through tempo runs, threshold efforts, and race-pace work while continuing to build volume. This phase develops the physiological and psychological fitness necessary to sustain marathon pace and prepares the body for peak training.
This article explains the purpose of the build phase, key physiological adaptations, typical structure and duration, essential workouts, and how to progress intensity safely while maintaining the aerobic base.
Definition
The build phase is the middle period of marathon training where runners add targeted intensity work on top of an established aerobic foundation. This phase bridges the gap between base building and peak training by progressively introducing marathon-specific efforts.
Key characteristics of the build phase:
- Introduction of structured intensity (tempo runs, threshold work)
- Continued volume progression or maintenance of base volume
- Development of lactate threshold and sustained pace efforts
- Increased mental and physical demands compared to base phase
- Balance between quality work and aerobic maintenance
Core principle: Layer intensity gradually onto a strong aerobic base; build specificity without sacrificing durability.
Purpose of the build phase
Raise lactate threshold
Goal: Increase the pace at which lactate begins to accumulate faster than the body can clear it
How:
- Tempo runs and threshold efforts train the body to clear lactate efficiently
- Sustained hard efforts improve buffering capacity
- Adaptation occurs at the edge of aerobic/anaerobic transition
Result: Higher sustainable race pace; ability to run faster before accumulating fatigue
Develop marathon-specific endurance
Goal: Train the body to sustain efforts near marathon pace for extended periods
How:
- Long runs with marathon-pace segments
- Steady-state runs at goal pace
- Progressive long runs finishing at race effort
Result: Confidence and physiological capacity to maintain marathon pace
Increase stamina and muscular endurance
Goal: Build the ability to sustain effort under accumulating fatigue
How:
- Longer tempo and threshold efforts (20-40 minutes)
- Marathon-pace runs extending to 60-90 minutes
- Long runs reaching 18-22 miles with quality components
Result: Muscles and cardiovascular system adapt to prolonged hard efforts
Refine pacing skills
Goal: Develop awareness of marathon effort and learn to execute pacing strategy
How:
- Practice running by feel at goal pace
- Learn to manage effort over varying terrain
- Build confidence in race-day pacing plan
Result: Better race execution and reduced risk of going out too fast
Continue aerobic development
Goal: Maintain and further develop aerobic base while adding intensity
How:
- Keep 75-80% of weekly volume at easy intensity
- Maintain frequency and long run consistency
- Balance quality with recovery
Result: Sustained aerobic fitness that supports intensity work
Physiological adaptations during build phase
Lactate threshold improvements
Adaptation:
- Increased lactate clearance rate
- Improved buffering of hydrogen ions
- Greater reliance on fat at moderate intensities (sparing glycogen)
Timeline: Noticeable improvements within 4-6 weeks of consistent threshold work
Performance impact: Higher sustainable pace for marathon distance
Enhanced muscular endurance
Adaptation:
- Muscle fibers become more fatigue-resistant
- Improved contractile efficiency under sustained load
- Greater glycogen storage and utilization efficiency
Timeline: Progressive development over 6-10 weeks
Performance impact: Ability to maintain form and pace in later miles of marathon
Cardiovascular system refinement
Adaptation:
- Continued stroke volume improvements
- Enhanced cardiac output at higher intensities
- Better oxygen delivery under stress
Timeline: Builds on base phase adaptations; further improvements over 6-8 weeks
Performance impact: Lower heart rate at race pace, greater aerobic power
Improved running economy at race pace
Adaptation:
- Neuromuscular system becomes more efficient at goal pace
- Reduced oxygen cost at marathon effort
- Better coordination and mechanics under fatigue
Timeline: Gradual refinement over build phase
Performance impact: Lower physiological cost of running marathon pace
Psychological resilience
Adaptation:
- Confidence from successfully completing quality workouts
- Greater mental toughness to sustain discomfort
- Experience managing fatigue and pushing through challenging efforts
Timeline: Develops throughout build phase
Performance impact: Mental strength to execute race-day strategy and push through difficult miles
Typical structure and duration
Duration
Beginner marathoners:
- 8-10 weeks
- More gradual intensity introduction
Intermediate marathoners:
- 6-8 weeks
- Moderate intensity progression
Advanced marathoners:
- 6-8 weeks
- May include higher volume and more frequent quality sessions
General principle: Build phase should be long enough to develop threshold and race-specific fitness without rushing adaptation
Weekly structure
Example week (intermediate runner, 45-50 miles/week):
- Monday: Rest or easy 4 miles
- Tuesday: Tempo run: 2 mile warm-up + 5 miles at threshold pace + 1 mile cool-down (total: 8 miles)
- Wednesday: Easy 6 miles
- Thursday: Easy 5 miles + strides
- Friday: Rest or easy 4 miles
- Saturday: Long run: 14 miles with final 6 at marathon pace
- Sunday: Easy 8 miles
Total: 49 miles Quality sessions: 2 (Tuesday tempo, Saturday long run with MP work) Easy volume: ~75-80%
Key features:
- 1-2 quality sessions per week
- Recovery days between hard efforts
- Continued emphasis on easy running
Volume progression
Option 1: Maintain base volume while adding intensity
- Keep weekly mileage stable at base phase peak
- Layer in quality work without increasing total volume
- Lower injury risk; conservative approach
Option 2: Gradual volume increase with intensity
- Increase volume by 5-10% over build phase
- Add intensity simultaneously but carefully
- Requires excellent recovery practices
Example progression (Option 2):
- Week 1: 42 miles (1 tempo)
- Week 2: 45 miles (1 tempo, 1 MP long run)
- Week 3: 48 miles (1 threshold, 1 MP long run)
- Week 4: 36 miles (recovery week)
- Week 5: 50 miles (1 tempo, 1 progressive long run)
- Week 6: 52 miles (1 threshold, 1 MP long run)
- Week 7: 54 miles (1 tempo, 1 race-pace long run)
- Week 8: 40 miles (recovery week)
Rationale: Gradual progression with built-in recovery prevents overtraining
Key workouts and components
Tempo runs (comfortably hard)
Purpose: Develop lactate clearance and sustainable hard effort
Intensity: 80-88% max heart rate; "comfortably hard" effort
Duration: 20-40 minutes continuous
Pace: Typically 25-35 seconds/mile slower than 5K pace, or 15-25 seconds slower than 10K pace
Structure:
- 10-15 minute warm-up
- 20-40 minute tempo effort
- 10 minute cool-down
Frequency: Once per week
Execution:
- Start controlled; settle into rhythm
- Effort should feel challenging but sustainable
- Should be able to speak short sentences (not conversational)
Example workout: 2 mile warm-up + 5 miles at tempo pace + 1 mile cool-down
Threshold runs (at lactate threshold)
Purpose: Train at the edge of aerobic/anaerobic transition
Intensity: 85-90% max heart rate; "hard but controlled"
Duration: 15-30 minutes continuous, or intervals totaling 30-40 minutes
Pace: Approximately 10K-half marathon race pace
Structure (continuous):
- Warm-up
- 20-30 minutes at threshold pace
- Cool-down
Structure (intervals):
- Warm-up
- 3-4 × 8-10 minutes at threshold with 2-3 minute recovery
- Cool-down
Frequency: Once per week (alternating with tempo runs)
Execution:
- Controlled effort throughout
- Focus on steady breathing and rhythm
- Should feel hard but not all-out
Example workout: 2 mile warm-up + 4 × 8 minutes at threshold (2 min jog recovery) + 1 mile cool-down
Marathon-pace long runs
Purpose: Practice running at goal marathon pace; build race-specific endurance
Structure (progression into MP):
- First 60-70% at easy pace
- Final 30-40% at marathon pace
Structure (alternating MP segments):
- Alternate easy miles with marathon-pace miles
- Example: 2 easy, 3 MP, 2 easy, 3 MP, 2 easy
Structure (continuous MP):
- Easy warm-up miles
- 6-10 miles continuous at marathon pace
- Easy cool-down
Frequency: Every 2-3 weeks, alternating with easy long runs
Total duration: 90-150 minutes
Execution:
- Begin long run relaxed and controlled
- Marathon-pace segments should feel manageable, not maximal
- Practice race-day fueling and hydration
Example workout: 14 miles total (6 easy + 6 at marathon pace + 2 easy)
Steady-state runs
Purpose: Sustained moderate effort; bridge between easy and tempo
Intensity: 75-82% max heart rate; "moderate" effort
Duration: 40-60 minutes
Pace: Between easy pace and tempo pace; close to marathon pace or slightly faster
Frequency: Once every 1-2 weeks
Execution:
- Warm up thoroughly
- Settle into steady, controlled rhythm
- Should feel like a moderately hard but sustainable effort
Example workout: 2 mile warm-up + 6 miles steady-state + 1 mile cool-down
Long runs (easy aerobic)
Purpose: Continue building endurance; maintain aerobic base
Intensity: 65-75% max heart rate; easy, conversational
Duration: 90-150 minutes
Frequency: Every week (alternating easy and quality long runs)
Execution:
- Maintain easy effort throughout
- Focus on time on feet, not speed
- Practice nutrition and hydration
Example workout: 16 miles at easy pace
Progression long runs
Purpose: Teach body to run strong when fatigued; simulate race conditions
Structure:
- First 60-70%: Easy pace
- Middle 15-20%: Moderate/steady pace
- Final 10-15%: Marathon pace or slightly faster
Duration: 90-150 minutes total
Frequency: Once every 3-4 weeks
Execution:
- Start conservatively
- Gradual, smooth acceleration through phases
- Finish feeling strong but not maximal
Example workout: 16 miles (10 easy, 4 moderate, 2 at marathon pace)
Strides and drills (continued from base)
Purpose: Maintain neuromuscular sharpness and running mechanics
Frequency: 2 times per week after easy runs
Execution:
- Same as base phase
- 4-6 × 15-20 seconds at 85-90% effort
- Full recovery between reps
Importance: Prevents loss of speed and reinforces efficient form
Training intensity distribution in build phase
75/20/5 or 80/15/5 principle
Low intensity (easy pace): 75-80% of weekly volume
- Easy runs, recovery runs, easy portions of long runs
Moderate intensity (tempo/threshold): 15-20% of weekly volume
- Tempo runs, threshold work, marathon-pace segments
High intensity (VO₂max): 0-5% of weekly volume
- Strides, optional short intervals (late build)
Rationale: Majority of volume remains easy to support recovery and continued aerobic development; targeted intensity develops race-specific fitness
Frequency and recovery management
Spacing quality sessions
Guideline: At least 48 hours between hard workouts
Typical pattern:
- Hard day (tempo or threshold)
- Easy day
- Easy day
- Hard day (long run with quality)
- Easy day
- Easy day or rest
- Easy day
Rationale: Allows full recovery and adaptation between quality efforts
Easy days must stay easy
Critical principle: Easy runs between quality sessions should be truly easy (60-70% max HR)
Common mistake: Running moderate effort on "easy" days, limiting recovery
Solution:
- Monitor heart rate
- Slow down deliberately
- Focus on feeling relaxed and loose
Recovery weeks
Frequency: Every 3-4 weeks
Volume reduction: 20-30% decrease in total mileage
Intensity: Reduce or eliminate one quality session; keep remaining session moderate
Purpose: Allow accumulated fatigue to dissipate; facilitate adaptation
Example recovery week: 36 miles with one easy tempo (instead of 50 miles with two quality sessions)
Strength training during build phase
Purpose
Goals:
- Maintain muscular strength and power developed in base
- Support running-specific strength for sustained marathon efforts
- Prevent injury as training stress increases
Frequency and timing
Frequency: 2 sessions per week (reduced from base phase if 3x/week)
Timing:
- After easy runs or on rest days
- Avoid heavy lifting day before quality running sessions
Focus areas
Lower body strength:
- Squats, Bulgarian split squats, lunges
- Deadlifts, Romanian deadlifts
- Calf raises, single-leg exercises
Core stability:
- Planks, side planks, anti-rotation exercises
- Dead bugs, bird dogs
Plyometrics (moderate volume):
- Box jumps, bounding
- Builds power and elastic energy return
Upper body (maintenance):
- Push-ups, rows
- Light weights to maintain balance
Volume and intensity
Volume: Moderate (2-3 sets × 6-10 reps for strength exercises)
Intensity: Moderate to moderately heavy
Caution: Avoid excessive volume or intensity that impairs running recovery
Mental and psychological development
Building confidence
How:
- Successfully completing quality workouts builds belief in fitness
- Marathon-pace runs demonstrate ability to sustain goal pace
Importance: Mental confidence translates to race-day execution
Practicing mental toughness
How:
- Pushing through discomfort in tempo and threshold runs
- Managing fatigue in late stages of long runs
- Executing workouts even when motivation is low
Result: Greater resilience to handle marathon challenges
Rehearsing race-day strategies
How:
- Practice pacing and effort management during MP runs
- Test fueling and hydration strategies
- Simulate race-day routines (pre-run meal, warm-up)
Result: Reduced anxiety and better execution on race day
Common mistakes in the build phase
Adding too much intensity too quickly
Problem: Jumping from zero quality work to multiple hard sessions per week
Consequence:
- Injury risk spikes
- Overtraining and burnout
- Incomplete recovery
Solution:
- Start with one quality session per week
- Add second session only after adapting (2-3 weeks)
- Progress intensity and volume gradually
Running easy days too hard
Problem: "Moderate" effort on days meant to be easy
Consequence:
- Chronic fatigue
- Poor quality on hard days
- Increased injury risk
Solution:
- Strict heart rate monitoring on easy days
- Embrace truly easy paces
- Separate "easy" from "moderate" clearly
Neglecting recovery weeks
Problem: Continuously increasing training load without planned recovery
Consequence:
- Accumulated fatigue
- Stagnant or declining performance
- Injury
Solution:
- Schedule recovery weeks every 3-4 weeks
- Reduce volume and/or intensity
- Trust the process
Overemphasizing marathon pace too early
Problem: Running every long run at marathon pace from start of build
Consequence:
- Limits total long run distance
- Excessive fatigue
- Reduces aerobic stimulus
Solution:
- Alternate easy long runs with quality long runs
- Begin with shorter MP segments (4-6 miles), progress to longer (8-10 miles)
- Include marathon-pace work in final 30-40% of run, not entire run
Skipping strength training
Problem: Dropping strength work to "save energy" for running
Consequence:
- Muscular imbalances worsen
- Injury risk increases as running volume and intensity rise
- Lost opportunity for performance gains
Solution:
- Maintain 2x/week strength sessions
- Adjust volume/intensity to avoid interference with running
- Prioritize key exercises (squats, single-leg work, core)
Ignoring warning signs of overtraining
Problem: Pushing through persistent fatigue, elevated resting HR, declining performance
Consequence:
- Injury
- Burnout
- Setback requiring extended recovery
Solution:
- Monitor daily markers (resting HR, sleep, mood, motivation)
- Take extra rest day if warning signs appear
- Adjust plan rather than rigidly following schedule
Monitoring progress during build phase
Workout performance markers
Improving times at same effort:
- Tempo pace getting faster at same RPE
- Marathon-pace segments feel easier
Better recovery between intervals:
- Heart rate drops faster during threshold interval recoveries
Longer sustained efforts:
- Able to extend tempo runs from 20 to 30 minutes
- Marathon-pace segments increase from 4 to 8 miles
Physiological markers
Lower heart rate at marathon pace:
- Track HR during MP segments on long runs
- Decreasing HR at same pace = improved fitness
Faster paces at threshold heart rate:
- Run by HR zones; increasing pace = improved lactate threshold
Subjective markers
Quality workouts feel more manageable:
- Tempo runs no longer feel overwhelming
- Confidence in executing marathon-pace segments
Better recovery day-to-day:
- Legs feel fresh on easy days
- Sleep quality remains good
Sustained motivation:
- Enjoying training despite increased demands
Transitioning from build to peak phase
Signs you're ready to progress
Threshold fitness established:
- Comfortable running tempo and threshold efforts
- Marathon pace feels sustainable
Volume capacity reached:
- Handling weekly mileage well
- Recovering adequately between sessions
No injuries or persistent issues:
- Aches and pains are minor and transient
Psychological readiness:
- Confidence in fitness
- Motivated to tackle peak training
How to transition smoothly
Maintain intensity, increase specificity:
- Continue tempo and threshold work
- Increase frequency of marathon-pace runs
- Add longer race-pace segments
Peak volume (or maintain):
- Reach highest weekly mileage in early peak phase
- Begin to taper volume in later peak phase
Increase mental rehearsal:
- Simulate race-day conditions more frequently
- Fine-tune pacing and fueling strategies
Continue monitoring:
- Stay vigilant for signs of overtraining
- Adjust as needed; peak phase is demanding
Sample build phase plans
Beginner (40 miles/week peak)
Duration: 8 weeks
Quality sessions:
- 1 tempo or threshold run per week (weeks 1-4)
- 1 tempo/threshold + 1 MP long run (weeks 5-8)
Long run progression:
- Start: 10 miles easy
- Mid-phase: 12-14 miles with 4-6 MP
- Peak: 16 miles with 6-8 MP
Intermediate (55 miles/week peak)
Duration: 8 weeks
Quality sessions:
- 1 tempo/threshold per week
- 1 quality long run every 2 weeks
Long run progression:
- Start: 14 miles easy
- Mid-phase: 16 miles with 6-8 MP
- Peak: 18-20 miles with 8-10 MP
Advanced (70 miles/week peak)
Duration: 8 weeks
Quality sessions:
- 1 tempo/threshold per week
- 1 quality long run per week or every 10 days
Long run progression:
- Start: 16 miles easy or progressive
- Mid-phase: 18-20 miles with 8-10 MP
- Peak: 20-22 miles with 10-12 MP
Summary
The build phase adds structured intensity to the aerobic foundation established during base training, developing marathon-specific fitness through tempo runs, threshold work, and race-pace efforts. Physiological adaptations include improved lactate threshold, enhanced muscular endurance, cardiovascular refinement, better running economy at race pace, and psychological resilience. The build phase typically lasts 6-10 weeks with 1-2 quality sessions per week while maintaining 75-80% of volume at easy intensity. Key workouts include tempo runs (20-40 minutes comfortably hard), threshold efforts (at 10K-half marathon pace), marathon-pace long runs, steady-state runs, and progression long runs. Volume may increase modestly or remain stable while intensity is added; recovery weeks every 3-4 weeks prevent overtraining. Common mistakes include adding intensity too quickly, running easy days too hard, neglecting recovery weeks, and overemphasizing marathon pace early in the phase. Strength training continues at 2x/week to maintain durability. Progress is monitored through improving workout performances, lower heart rate at marathon pace, and subjective markers of recovery and confidence. Transitioning to peak phase occurs when threshold fitness is established, volume capacity is reached, and both physical and psychological readiness are confirmed.