12-Week Half Marathon Training Plan: Science-Backed Structure with VDOT Paces

The half marathon is the most physiologically demanding race distance per minute of effort for most recreational runners. A marathon is largely an aerobic and metabolic challenge — can you sustain fat burning and maintain form over 42 km? A half marathon is that, plus a significant speed challenge: you race at approximately 85% of your VO2max for 90 minutes to two hours, hovering just above your lactate threshold for the entire race.

That distinction shapes everything about how to train for it.

My first half marathon I ran on general fitness — lots of easy mileage, no structured quality work. I finished in 2:04 and felt I'd raced hard from km 8 onward. The second cycle I added two threshold sessions per week and a proper periodization structure. I ran 1:51. The fitness base was similar; the lactate threshold work was the difference.

How the half marathon differs from the marathon

Understanding the physiology of the half marathon shapes every training decision:

Intensity: Half marathon race pace sits at ~85% VO2max — just above lactate threshold for most intermediate runners. Marathon race pace sits at ~75–80% VO2max, below threshold. This means half marathon training demands more threshold-specific work and less pure aerobic volume than marathon training.

Long run: The critical long run for a half marathon is 18–22 km — not the 30–32 km required for marathon prep. You need enough aerobic endurance to hold form for 90–120 minutes, not four-plus hours.

Taper: Half marathon taper is 1–2 weeks, not 2–3. Less accumulated fatigue means less time needed to shed it.

Speed: Half marathon training includes more R-pace repetition work than marathon training. Speed matters here in a way it doesn't for the marathon, where economy at race pace is more important than peak velocity.

Who this plan is for

This plan is designed for intermediate runners who:

• Have a recent race result to calculate VDOT
• Are running 35–50 km/week consistently before starting
• Have finished at least one half marathon or several 10K races
• Are targeting a specific time goal, not just a finish

If you're below 25 km/week, build to 35 km/week over 4–6 weeks before starting week 1.

Finding your training paces

Calculate your VDOT from a recent race result. If your most recent race was a 10K, use that. Half marathon results work equally well.

Use your VDOT to set paces before week 1. If you run a tune-up race at week 5–6, recalculate and update paces for the remainder of the plan.

The three phases

Phase 1 — Base (weeks 1–4)

Goal: Aerobic foundation and connective tissue adaptation
Volume: Build from starting base to ~65% of peak
Intensity: Primarily E pace, strides 2× per week, no threshold work until week 3

Half marathon runners often want to start threshold sessions immediately. Resist this. Four weeks of aerobic base work raises your LT1 (aerobic threshold) gradually and prepares the musculoskeletal system for the harder work ahead. Connective tissue adapts more slowly than cardiovascular fitness — skipping base phase is the most reliable path to a week-8 injury.

CTL target: Build at +3–4 points/week. TSB should stay above −10.

Sample week (week 3):

Phase 2 — Build (weeks 5–9)

Goal: Raise lactate threshold — the primary performance driver for half marathon
Volume: Peak volume — 55–70 km/week for most intermediate runners
Intensity: E + T + R pace. Two quality sessions per week: one threshold, one speed

This is where the half marathon plan diverges most sharply from marathon preparation. Threshold work — cruise intervals at T pace — becomes the central training stimulus. Lactate threshold is the ceiling of your sustainable race pace, and the half marathon is run right at or just above it. Raising your T pace raises your half marathon pace.

The second quality session uses R-pace repetitions: short, fast efforts (200–400m) at mile pace or faster with full recovery. These improve running economy and neuromuscular efficiency — the speed component that the marathon plan doesn't prioritise.

Easy days must be genuinely easy. Two hard sessions per week is a significant stimulus load. If easy runs creep toward M pace, you're accumulating ATL without the recovery needed to absorb the quality work.

CTL target: +3–5 points/week. TSB oscillates between −5 and −20. Recovery weeks (weeks 7) should allow TSB to return toward −5.

Sample week (week 7):

Phase 3 — Peak and taper (weeks 10–12)

Goal: Maximum CTL, then shed fatigue → TSB +8 to +15 on race day
Volume: Week 10 at peak, week 11 reduced 30%, week 12 race week
Intensity: Week 10 hardest quality sessions; week 11 one short quality session; week 12 strides only

The half marathon peak phase is compressed compared to marathon prep — two weeks rather than three. This reflects the lower accumulated training load: with 12 weeks of preparation at 55–70 km/week, CTL peaks lower than after a 20-week marathon cycle, and less taper time is needed to shed ATL.

Week 10: highest quality sessions of the cycle — longest T-pace session + a race-simulation workout at goal half marathon pace.

Week 11: volume drops 30%. One short quality session early in the week (3 × 1 km at T pace). Rest or easy running after Wednesday.

Week 12 (race week): easy running only, 30–40 minutes per session, with 4–6 strides Wednesday. Rest Thursday or Friday. Race Saturday or Sunday.

Race day TSB target: +8 to +15. Slightly lower than the marathon target because the shorter taper produces less ATL drop. If TSB is below +5, note it — your next half marathon cycle needs a longer taper window or lower peak volume.

Sample week (week 10 — peak):

Weekly volume overview

The threshold priority

Every half marathon plan should answer the same question: what raises half marathon performance most directly?

The answer is lactate threshold. Your half marathon race pace is approximately your T pace minus 10–15 seconds/km for most intermediate runners. Raise your T pace and your race pace rises proportionally.

This means the most valuable sessions in this plan are the Tuesday cruise intervals in weeks 5–10. Not the long run. Not the speed work. The threshold sessions are where the performance gain comes from.

Practical implication: if you have to cut a session in a given week, cut the R-pace speed work or a mid-week easy run before you cut the T-pace session. The threshold work is load-bearing.

How this differs from generic half marathon plans

Most generic half marathon plans prescribe paces based on your goal finish time. If you're targeting sub-1:50, your threshold pace is X — regardless of whether you're currently capable of running T-pace sessions at that intensity.

This plan derives every pace from your current VDOT. A runner targeting sub-1:50 who is currently at VDOT 47 has a T pace of ~5:30/km. A generic sub-1:50 plan might prescribe tempo work at 5:12/km — 18 seconds/km too fast. Sessions at that intensity produce excess fatigue, increase injury risk, and stimulate less LT adaptation than correctly-paced work would.

Adaptive plans extend this further — recalibrating not just paces but session lengths and taper timing based on your actual CTL, ATL, and TSB each week. When you miss sessions, the plan rebuilds conservatively. When your VDOT rises mid-cycle, every pace updates automatically.

Ready to step up to the marathon?

The half marathon is the ideal platform for building the aerobic base and lactate threshold fitness that marathon training demands. After two or three half marathon cycles at improving VDOT scores, you'll have the aerobic infrastructure — and the training discipline — for a serious 16-week marathon plan.

The physiological progression is not incidental: the T-pace work that drives half marathon performance is the same foundation that makes Daniels' and Pfitzinger's marathon methods work.

Twelve weeks is enough to build meaningful half marathon fitness when the intensity is right, the easy runs are genuinely easy, and the threshold sessions are the priority. The paces matter as much as the structure — and both need to match where you actually are, not where you want to be.

Frequently Asked Questions

How many weeks do you need to train for a half marathon?

12 weeks is sufficient for intermediate runners with a base of 35–45 km/week. It allows 4–5 weeks of base and build work, 3–4 weeks of quality-focused peak training, and a 1–2 week taper. Beginners or runners coming off a long break should use a 16-week plan to build adequate aerobic foundation before quality work begins.

What pace should I run for half marathon training?

Every pace in half marathon training should be derived from your VDOT score, calculated from a recent race result. Your easy runs should be at E pace (genuinely conversational), your threshold sessions at T pace (comfortably hard, sustainable for ~60 minutes), and your long runs at E pace with optional M or T pace segments. Training at generic paces based on your goal time — rather than your current fitness — is the most common reason half marathon training fails.

What is the key workout for half marathon training?

Threshold work — cruise intervals and tempo runs at T pace — is the primary quality stimulus for half marathon performance. The half marathon is run at approximately 85% of VO2max, which sits just above lactate threshold for most runners. Raising your lactate threshold directly raises the ceiling of your sustainable race pace. Most intermediate runners need 1–2 threshold sessions per week in the build phase, totalling no more than 10% of weekly volume at T pace.