The half marathon is the best-designed race in running. It is long enough that aerobic fitness decides the outcome, short enough that you can train for it without reorganising your life, and forgiving enough that a slightly imperfect build still produces a good day.
It is also the distance where threshold training pays the highest dividend, because race pace sits close to the intensity that threshold work directly improves.
What a 12-week half marathon block looks like
Shorter than a marathon block and less punishing, but the same shape: waves of building volume, a lighter week to absorb it, then a taper into race day.
- Peak week
- 40 milesweek 9
- Race week
- 15 milesabout 38% of peak
- Base Volume grows, nearly all of it easy
- Build Volume holds, quality increases
- Peak Longest runs, most race-specific work
- Taper Volume drops, freshness returns
Illustrative for a runner peaking near 40 miles a week. The shape is what matters, not the numbers: your own peak depends on your base, your history, and the hours you actually have.
How long you need
Ten to fourteen weeks for most runners with a base of regular running. Twelve is the comfortable default. If you can already run 6 or 7 miles without it being an event, ten weeks is enough. If you are starting from a few miles at a time, take the longer end and spend the first block simply running more.
Unlike the marathon, you do not need the extra weeks as insurance against the distance itself. You need them to fit in enough quality work.
The sessions that matter
The long run. Build toward 10 to 14 miles. Past that you get diminishing returns for a half, and the recovery cost starts eating the quality sessions that matter more here. Keep most of it easy, and in the second half of the block, finish the last 2 or 3 miles at goal race pace so you learn what it feels like on tired legs.
Threshold work. The highest-value session for this distance. Tempo runs of 20 to 40 minutes, or cruise intervals such as 3 or 4 sets of 8 to 10 minutes with short recoveries. Half marathon pace sits just a little slower than threshold for most trained runners, so this work moves race pace directly.
Easy running. Everything else. It should be the clear majority of your week, and it should be genuinely easy. See understanding your training paces if your easy pace has crept up.
Why threshold, not intervals
Short VO2 max intervals are useful, but for a race lasting 80 to 120 minutes, the ability to hold a strong steady effort matters more than your ceiling. If you only have room for one hard session a week, make it a threshold session.
Setting a goal
Run a 5K or 10K in the block and convert it with the race time predictor. Riegel is at its most reliable across this range, so a recent 10K gives a genuinely useful half marathon estimate, unlike the same prediction stretched to a marathon.
Turn the target into per-mile numbers and splits with the running pace calculator.
Race day
Start slightly slower than goal pace for the first two miles. The half is short enough that you can make up a few seconds and long enough that going out hot costs you minutes.
Fuelling is simpler than a marathon but not nothing. Most runners want something around the 45 to 60 minute mark, and anyone over about two hours should plan properly. Practise it in training rather than improvising on the day.
Common mistakes
- Training it like a short marathon. Piling on long runs at the expense of threshold work is the most common error at this distance.
- Easy days too hard. The usual culprit, and it costs you the threshold sessions that decide this race.
- No goal-pace running. If you never run at race pace in training, race pace will feel foreign on the day.
- Over-tapering. A half needs about 10 days of reduced volume, not three weeks. Keep some intensity in.
Going further
If the half is a step toward a longer goal, how to train for a marathon picks up where this leaves off. If you are sharpening down instead, how to train for a 10K leans harder on speed.
Whichever direction you go, the hard part is not knowing the sessions. It is fitting them around the weeks where work, travel, and fatigue get a vote. That is the part Vaast handles, rebuilding the block around what you actually managed rather than what the plan assumed.