Half marathon

A half marathon is 21.0975 kilometres, or 13.1094 miles — exactly half the marathon distance, which is where the name comes from and very nearly where the resemblance ends. World Athletics recognises it as a road running discipline in its own right with its own championship history, and it is by a wide margin the most popular serious race distance in Britain.

The reason it is not simply a shorter marathon is that the limiting factor changes. A marathon is decided by whether you can keep going at all, which is why fuelling and pacing discipline dominate it. A half is short enough that a reasonably prepared runner will finish, and long enough that how fast is a real question — so it is a race you run rather than an event you complete, and the tactical problem is much closer to a 10k than to a marathon.

That has a practical consequence people find liberating once it is stated. A bad half costs you a fortnight; a bad marathon costs a season. You can race two or three halves a year without the whole calendar bending round them, and you can afford to get one wrong and learn something. Most runners who try both end up here.

The distance

21.0975km / 13.1094 milesExactly half the marathon distance.

What decides it

Pacing, mostlyFitness gets you round; pacing decides whether it was any good.

Recovery

Days rather than weeksThe reason it can be raced several times a year.

Sensible prerequisite

Comfortably running 8–10kmRather than "beginner", which most plans mean as beginner at the distance.

Almost every published plan assumes a starting point it does not state: that you already run comfortably three times a week and can cover 8–10km without it being an event. Plans described as "beginner" usually mean beginner at the distance. Starting from a lower base is the commonest reason a block falls apart, and it falls apart through injury or simply through the sessions not being completable.

The order that works, without a schedule attached:

1. 1

   Step 1: Build a habit before building distance

   Three runs a week for a couple of months, comfortably. This is the base every plan assumes and none provides, and it is where the injury risk is actually managed.

   You will know it worked when: Running is routine rather than a project.

2. 2

   Step 2: Race something shorter first

   A parkrun or a 10k. You learn pacing, race-morning logistics and what your stomach does under pressure at a distance where getting it wrong costs twenty minutes.

   You will know it worked when: The half is not also your first race.

3. 3

   Step 3: Choose the event before worrying about the training

   Date, terrain, likely weather, cut-off, and how you get there. A flat local race in spring is materially easier than a hilly destination one in August, and no amount of preparation compensates for that choice.

   You will know it worked when: You are preparing for a specific day.

4. 4

   Step 4: Extend one run a week, gradually

   The long run is where the adaptation happens. Most approaches build it towards but not beyond race distance, and most cap it somewhere between 16 and 20km — which surprises people and is close to universal.

   You will know it worked when: A long run that makes the distance unremarkable.

5. 5

   Step 5: Keep most of the rest easy

   The majority of weekly mileage run slower than feels productive. Running easy runs too fast is the most common training error in the sport and it is invisible until the block collapses.

   You will know it worked when: Enough recovery to absorb the long run.

6. 6

   Step 6: Rehearse the race in training

   The shoes, the socks, the kit, the breakfast, and any drink you intend to take. Nothing on race day should be new — the phrase every runner learns eventually is that nothing new happens on race day.

   You will know it worked when: No unknowns left for the morning.

If there is one thing to take from this page, it is that the half is decided in the first three kilometres and almost nobody believes that until it has happened to them twice.

The mechanism is straightforward. At half marathon effort you are running close to a pace you can sustain for around ninety minutes to two hours, and the margin between "sustainable" and "not sustainable" is narrow. Starting fifteen seconds a kilometre too fast feels almost free for twenty minutes and is unrecoverable by fifteen kilometres — you do not simply slow down to the correct pace, you slow down well below it. The time lost in the second half is several times the time gained in the first.

The opposite error is much cheaper. Starting slightly conservatively costs a few seconds and buys the ability to run the last five kilometres, which is where a good half is actually made. This asymmetry is why nearly every experienced runner’s advice reduces to the same sentence: go out slower than you want to.

How the splits usually work:

• Even splits — Running each half of the race in roughly the same time. The default target and what most well-executed halves look like.
• Negative splits — Second half faster than the first. Widely regarded as the ideal and considerably rarer than people claim, because it requires real restraint early.
• Positive splits — Second half slower. Extremely common, usually by a large margin, and almost always caused by the first three kilometres rather than by the last three.
• The crowd effect — Everybody around you is also going too fast at the start, and the pace feels correct because it is the pace of the field. This is why watching your own watch in the first two kilometres matters more than at any other point.
• Course profile — A pace target set on flat ground does not survive a hill. Run the effort rather than the number on climbs, and let the pace come back on the descent.

Pace measurement itself has a wrinkle worth knowing. A GPS watch will nearly always record a half marathon course as slightly long, because it samples a path that cuts fewer corners than the measured line does, and because signal in a city drifts. Do not conclude the course was mismeasured — see the section on how courses are certified — and do not chase the watch’s pace over the official markers.

The half is unusual among race distances in being a genuine destination rather than a staging post, and a great many runners spend a decade here quite deliberately. The reasons are practical: the training fits around a life, the recovery is measured in days, and there is a race somewhere near you most months of the year.

The usual directions:

• Race it properly — Most first halves are completions. The second and third are where pacing gets learned, and the improvement between them is usually larger than any training change produces later.
• Get faster rather than longer — Adding structured faster running — intervals, tempo work — is what moves a half time, and it is a smaller time commitment than adding distance.
• Go off-road — Trail halves, multi-terrain events and fell races, where the clock stops meaning anything and the terrain decides everything. A common and happy landing place.
• Step up to the marathon — The obvious progression and a much larger commitment — a training block that reorganises the calendar and a recovery measured in weeks. Worth doing knowingly rather than by default.
• Sideways into triathlon — Where the running is one third of the problem and the equipment bill is an order of magnitude larger.
• Join a club — The single biggest improvement available to most runners, and the cheapest. Structured sessions with other people beat self-directed training for almost everybody.

• Shoes — The only purchase that genuinely matters, and the one most over-theorised. A pair that fits, feels good on a long run and has not done several hundred kilometres already. Gait analysis at a running shop is a reasonable free service and its recommendations are not a science.
• Socks — Technical rather than cotton, and worn in. Blisters over two hours are a genuine race-ruiner and cost £8 to prevent.
• Something that does not chafe — Whatever you have run in repeatedly. Two hours finds every seam, and the fix is anti-chafe balm, not a new garment on the day.
• A watch, optionally — £80–£300, and useful mainly for the first three kilometres when your judgement is worst. A plain stopwatch and the kilometre markers do the same job.
• A discardable layer — An old jumper or a bin bag for the start pen. The single highest-value item on race morning in a British winter.
• A hydration carrier, rarely — Most road halves have aid stations often enough that carrying a bottle is unnecessary. Trail events are a different matter and often mandate kit.

The thing not to buy is anything new in the fortnight before the race. That includes the shoes you have been meaning to replace, the vest from the expo, and any product somebody at the start line recommends. Nothing new on race day is a cliché because it is a rule.

More halves are spoiled by admin than by fitness. The failures are boring and entirely preventable, and they cluster in the two hours before the start.

What actually goes wrong:

• Arriving late — Parking at a large event takes far longer than the website implies, and road closures start earlier than you expect. The runners jogging to the start line have already lost the race they wanted.
• Starting in the wrong pen — Too far forward and you get dragged out fast; too far back and you spend three kilometres weaving. Enter the honest predicted time rather than the aspirational one.
• New kit — A race t-shirt worn on the day, new socks, a different sports bra. Chafing over two hours is a genuine injury and it is entirely self-inflicted.
• Breakfast roulette — Eating something you have never eaten before a run. Whatever you have practised is the right answer, including nothing if that is what you practised.
• Toilet queues — The single most underestimated element of race morning. Budget twice as long as seems reasonable.
• Dressing for the start — Standing around cold, then overdressing for it. The convention is to dress for about ten degrees warmer than it is, with an old layer to discard at the start.

Aid stations are worth understanding before you meet one at speed. They are typically placed every three to five kilometres, they are crowded, and the ground around them is wet and littered with cups and bottles. The conventions are simple: take the drink from a volunteer’s hand as you pass rather than reaching across people, move to the outside if you are going to walk, and drop your cup at the end of the station rather than in the middle of the crowd. Over this distance most runners need considerably less than they take — but the decision about what and how much is individual, and this page gives no figures.

A personal best only means something if the course was the distance it claimed, and the machinery that guarantees that is genuinely interesting and almost never explained to the people relying on it.

In the United Kingdom, a licensed road race advertised over a specific distance must hold a certificate of course measurement, issued through the Association of UK Course Measurers, and the certificate is submitted with the event licence application — a licence is not granted without one. Measurement is done by riding the course on a bicycle fitted with a counter that records front-wheel revolutions, calibrated before and after against an accurately surveyed calibration course. Certificates typically remain valid for a number of years provided the course is unchanged.

What follows from that, practically:

• Check the race is licensed and measured — Most are, and the good ones say so on the entry page. An uncertified "half marathon" may be any distance at all, which matters if you care about the time.
• Your watch will read long, and that is normal — The measured line follows the shortest legal route round every bend. You do not run that line, and GPS drifts in cities. A watch reading 21.3km on a certified course is measuring you, not the course.
• Multi-terrain events are measured differently — Trail and off-road events often cannot be measured by the standard method and carry a statement rather than a certificate. Comparing a trail half to a road half is comparing two different things.
• Chip time and gun time are not the same — Gun time runs from the start signal; chip time from when you crossed the line. In a large field the difference can be several minutes, and results usually publish both.
• Course records and personal bests need certified courses — This is what the whole system exists for. It is also why a spectacularly fast time on an unlicensed race means very little.