The short answer
A half marathon is 21.0975 km (13.1 miles). World Athletics' technical rules define the standard road-race distance. Training should connect enough easy running, one controlled quality purpose, a gradually longer run, recovery and race-day rehearsal to cover that distance without turning every week into a test.
Choose the shortest runway your current base can absorb. If consistent running is new, use extra weeks to establish the habit before race-specific work. More time is useful only when it makes progression calmer and missed weeks recoverable.
Choose 10, 12, 14, 16, 18 or 20 weeks
| Runway | A reasonable starting fit | Choose more time when |
|---|---|---|
| 10 weeks | You already run consistently, have a repeatable long run and mainly need race-specific structure | Recent weeks are inconsistent or the long run still creates several recovery days |
| 12 weeks | Your base is steady and the schedule can support one quality day plus one long-run focus | Work, travel or family demands frequently disrupt the week |
| 14 weeks | You want a moderate build with room to learn pacing and practise hydration | A return from time off needs a gentler first month |
| 16 weeks | You are building durability while keeping progression conservative | Easy frequency, strength or recovery routines are not yet reliable |
| 18 weeks | You need a longer base before specific threshold and race-effort work | Stress or sleep makes shorter blocks fragile |
| 20 weeks | You are new to consistent distance running or want the most gradual listed runway | Pain, illness or a severe schedule constraint needs qualified help or a later race—not simply more weeks |
Use the training start-date calculator with the actual race date. If you are still moving from shorter running, the 5K-to-half-marathon bridge can own the foundation phase before one of these blocks.
Readiness screen: find the limiter
Rate each area red, amber or green. Red means unsafe or not established, amber means possible but inconsistent, and green means repeatable without damaging the next training days.
| Area | Green evidence | Red flag to solve first |
|---|---|---|
| Frequency | Easy running fits most weeks without catch-up sessions | Each week restarts after soreness, exhaustion or schedule collapse |
| Long run | Duration grows gradually and the next days remain usable | One long run erases several sessions or pain changes movement |
| Quality | Controlled work finishes with stable form and breathing | Every workout becomes a time trial or requires repeated make-up rest |
| Recovery | Sleep, food and easy days support the next key purpose | Persistent fatigue, worsening mood, illness or declining easy-run tolerance |
| Schedule | The plan survives normal work, family and travel pressure | It works only during an unrealistically perfect week |
The first red area owns the next decision. Do not use a faster workout to solve an inconsistent base, and do not shorten recovery to compensate for a missed run.
A repeatable half-marathon week
Give each session one job:
- two or more easy aerobic runs at conversational effort;
- one controlled threshold, hill or race-effort purpose when recovery is stable;
- one gradually longer easy run, sometimes with a bounded steady finish;
- one or two short strength sessions that do not ruin the next run;
- at least one real rest or very easy day.
Three running days can work when they protect an easy run, one controlled quality purpose and the long run. Additional days should usually add easy volume, not another hard session. Physical Activity Guidelines for Americans provides general activity guidance; it does not prescribe an individual half-marathon load.
Use pace as feedback, not a verdict
Begin with the talk test and controlled effort. Then use the running pace calculator and race-time predictor to explore scenarios from recent evidence. Neither tool predicts weather, hills, congestion, health or the effect of an interrupted build.
For a time goal such as sub-2 hours, use the sub-2 half-marathon decision guide to test whether current training supports the target. Do not turn the goal pace into the pace for every run.
Progress the long run without proving the race
Start from the longest easy duration you already recover from. Increase one variable at a time: duration, terrain, a controlled finish or fueling practice. Cut the session back when form deteriorates, pain changes movement or fatigue persists into the next key day.
You do not need to race a half marathon in training to know whether the plan is working. A useful long run answers a question about durability, pacing, clothing, hydration or pre-run food without erasing the following week.
Practise hydration and pre-run food
Use longer runs and selected workouts to test breakfast timing, fluid access and any carbohydrate you expect to use. Record the product, amount, conditions, effort and gastrointestinal response, then change one variable at a time. Do not introduce an unfamiliar race-week plan.
Generic guidance cannot determine individual fluid needs or diagnose repeated gastrointestinal symptoms, dehydration or hyponatremia. Confusion, fainting, chest symptoms or severe breathlessness require urgent appropriate care.
When real life interrupts the plan
- One missed run: leave it missed; do not double the next day.
- Several missed days: resume with easy running and the original recovery spacing.
- A missed long run: continue from a duration you can still recover from rather than jumping ahead.
- A disrupted week: keep at most the highest-value quality or long-run purpose, not both at any cost.
- Two weeks away: use the return-to-training decision guide before restoring intensity.
- A longer disruption: lengthen the runway or choose a later race instead of compressing the block.
The plan should change when the evidence changes. Guilt is not training data.
Race-week decisions
Reduce load while preserving some familiar rhythm. Confirm the current organizer instructions, start time, course, aid stations, weather policy and transport. Use the taper checklist, prepare one conservative pacing plan and write the conditions that would make you slow down.
Evidence and safety boundaries
Official sources establish the race distance and organizer facts. Research informs broad activity, endurance, strength and hydration principles. Neither can choose your safe load, diagnose pain or guarantee a finish.
Stop and seek appropriate care for chest pain, fainting, confusion, acute injury, severe or unusual breathlessness, or symptoms that feel dangerous. Persistent pain that changes movement needs assessment rather than another training workaround.
Turn the runway into one plan
Use the free half-marathon training plan PDF when you need a printable view, then let completed work and recovery decide what happens next.
26weeks.ai keeps the half-marathon finish line visible while adapting training around the days life changes the plan. Start training now.