12-Week Half Marathon Training Plan: Week-by-Week Guide
Twelve weeks is the standard build for a first or early half marathon. It's enough time to develop the aerobic base and long run fitness you need for 21.1km — without asking more of your body than it can adapt to in a short window.
This guide gives you the complete structure of a 12-week half marathon plan: the week-by-week framework, how the long run progresses, what the different session types do, and how to manage the inevitable disruptions.
Prerequisites: what this plan assumes
This plan works if you're currently running at least 15–20km per week and can complete a 5K without stopping. If you're not there yet, spend 6–8 weeks building that base before starting this plan.
Starting half marathon training without a running base is the most common reason beginners get injured before race day.
The four phases of a 12-week plan
A well-structured half marathon training plan isn't twelve identical weeks. It periodises the training load across four phases:
| Phase | Weeks | Focus | Long run range |
|---|---|---|---|
| Base | 1–3 | Establish routine, easy aerobic work | 8–11km |
| Build | 4–7 | Increase volume, introduce some tempo | 12–16km |
| Peak | 8–10 | Highest mileage weeks, longest runs | 17–20km |
| Taper | 11–12 | Reduce volume, maintain sharpness, race | 12km → Race |
Week-by-week long run progression
The long run is the keystone of half marathon training. Every other session supports it; this is the one session you cannot skip.
| Week | Long run | Notes |
|---|---|---|
| 1 | 8km | Easy pace throughout — establish the habit |
| 2 | 10km | Still easy — don't race this |
| 3 | 11km | Recovery week: drop to 8km if fatigued |
| 4 | 13km | First real step into unfamiliar territory |
| 5 | 14km | Carry water and nutrition from here |
| 6 | 15km | Recovery week: 12km if tired |
| 7 | 16km | Include race-pace miles in the final third |
| 8 | 18km | Peak phase begins — take care of recovery |
| 9 | 19km | Longest effort before race; run by feel |
| 10 | 20km | Last long run — easy pace, full race distance |
| 11 | 12km | Taper begins — don't be alarmed by heavy legs |
| 12 | Race day — 21.1km | Trust the training |
Every third week is a recovery week — the long run drops slightly and total volume decreases. This isn't wasted training; it's when adaptation happens. Skipping recovery weeks is how runners arrive at the start line injured or exhausted.
You're not in trouble. One shortened long run in week 9 won't meaningfully affect your race fitness — especially if the preceding weeks went well.
| Metric | Status | Verdict |
|---|---|---|
| Long runs completed (wks 1–8) | All or mostly complete | ✅ Strong foundation |
| Week 9 long run | 16km instead of 19km | ⚠️ Short, but not critical |
| Weeks remaining | 3 (incl. taper) | ✅ Enough to recover |
| Race impact | Minimal | ✅ Fitness is already built |
What matters now:
- ✅ Don't try to compensate — adding extra miles this week or next week will do more harm than a cut-short long run
- ✅ Complete week 10's 20km run at easy pace — this is your last long run before the taper, and it matters more
- 🔴 Investigate why it felt bad — poor sleep, under-fuelling, too-fast pace, or accumulated fatigue? Address the cause
Marathon and half marathon fitness is built over months. One difficult run three weeks out doesn't erase it.
Real PacecraftAI response — actual output from the app
Session types: what each one does
Easy runs
The majority of your training — 70–80% — should be easy runs at a pace where conversation is comfortable. These build your aerobic base, improve fat oxidation, and allow recovery between harder efforts. Running them too fast is the most common mistake in half marathon training.
Tempo runs
Introduced in the build phase, tempo runs are sustained efforts at "comfortably hard" pace — faster than easy, slower than race pace. A typical session: 10-minute warmup, 20–30 minutes at tempo, 10-minute cooldown. They raise your lactate threshold — the pace at which your body starts accumulating lactic acid — which directly improves your half marathon time.
The long run
Covered above — the cornerstone. Always easy pace, always with adequate nutrition and hydration for anything over 60 minutes.
Training load: the numbers behind the plan
A 12-week beginner half marathon plan typically builds from around 25–30km per week to a peak of 45–55km per week, before tapering to 25–30km in the final week before the race.
The critical rule: weekly volume should not increase by more than 10% in any single week. Faster increases spike your acute training load relative to your chronic load — a ratio tracked by serious coaches and AI running coaches alike as one of the strongest predictors of injury.
Almost certainly not. The urge to add more mileage mid-plan is normal — but acting on it is usually where half marathon training goes wrong.
| Metric | Week 6 status | Flag |
|---|---|---|
| Weekly volume | On target (~35km) | ✅ Right where it should be |
| ACWR | 1.12 | ✅ Healthy build rate |
| Long run | 15km (wk 6 target) | ✅ Progressing correctly |
| Feeling "not enough" | Common in week 6 | ⚠️ Adaptation lag — trust the process |
- 🔴 Don't add volume — week 6 feeling easy is the sign the plan is working, not that you need more
- ✅ The hard weeks are coming — weeks 8–10 will not feel like too little
- ✅ Use the easy days for real recovery — sleep, nutrition, and stress management matter as much as miles
Fitness from week 6 training doesn't show up until weeks 8–10. You're building something you can't feel yet — that's normal, and it's working.
Real PacecraftAI response — actual output from the app
The taper: weeks 11–12
The taper is where first-time half marathoners feel most anxious. Reducing mileage feels counterintuitive — you're worried about losing fitness right before race day.
You won't lose fitness. The taper allows your body to fully absorb all the training you've done and arrive at the start line fresh. Your legs will feel heavy in week 11 (normal — glycogen stores are refilling). By race morning, you should feel good.
What not to do during the taper:
- Don't add extra sessions "to feel more prepared"
- Don't try a long test run to prove you're ready
- Don't try new foods, shoes, or gear
- Do sleep well, eat well, and trust the twelve weeks of work
Race day pacing
The most important piece of half marathon race day advice: start slower than you think you should. The crowd energy will make your target pace feel easy in the first kilometre. It's a lie. The runners passing you in the first 5km will be walking in the last 3km.
A simple race day strategy for first-timers:
- Km 1–5: Deliberately easy — 15–20 seconds per km slower than target pace
- Km 6–15: Settle into target race pace; stay comfortable
- Km 16–21: Assess. If you feel strong, pick it up. If not, maintain
Twelve weeks of consistent training is enough to finish a half marathon well. The plan works — if you trust it and follow it consistently. Start your plan free →
If you want a plan that adjusts when life disrupts your training, monitors your load to keep you injury-free, and answers your questions in real time — an AI running coach builds and manages all of that automatically. Your only job is to run.