A 100 mile race is a different event from a long ultra. Nearly everyone walks a big share of it, most runners see a full night, and the people who finish are usually the ones who solved problems early rather than the ones with the best marathon time. Here's how to get to a UK start line properly prepared.
Are you ready for it?
Most runners get here through a 50 miler or a hard 100K. Some races insist on it. Centurion's South Downs Way 100, for example, asks for a finish of at least 50 miles inside 15 hours, and the organiser can refuse entry without one. Even where nothing is required, a long race in the six months beforehand is the best test you can set yourself: it shows you what your stomach, feet and kit do after 10 hours, while there's still time to change them.
Choose the right race
For a first attempt, three things matter most.
Time limit. UK 100s range from about 24 hours to well over 30. The more hours you have, the more walking and stopping you can afford.
Course. Flat canal and river paths suit people who can keep moving, but they use the same muscles for hour after hour. National trails such as the South Downs Way add climbing and bigger views. Mountain 100s like the Lakeland 100 take far longer and need navigation skills.
Support. Regular aid stations, drop bags and crew access let you carry less and fix more. Look for a race with checkpoints close together and a clear course, ideally waymarked or on a national trail.
The UK 100 mile calendar lists every race we have, with dates and entry links. Good places to start reading include the Thames Path 100, South Downs Way 100, North Downs Way 100 and Autumn 100. Popular races can fill early, so check when entries open.
How long to train
Plan on 16 to 24 weeks of specific training on top of a solid base. If you've run plenty of ultras, the shorter end works. If you're stepping up from a 50K or a single 50 miler, give yourself 20 to 24 weeks, and don't start that block from scratch: build consistent running for two or three months first.
Think in hours, not miles. A common pattern is 8 to 12 hours a week in the main block, rising to 10 to 14 hours in peak weeks, with an easier week every third or fourth week. Mileage varies by runner and terrain, though about 50 miles a week is often quoted as a typical figure for finishing inside the limit. If you're short of time, protect the long runs and drop the short junk miles.
Long runs and back-to-backs
The session that defines 100 mile training is the back-to-back weekend: a long run on Saturday and another on Sunday on tired legs. Something like 3 to 5 hours on the first day and 2 to 4 hours on the second is plenty. Keep both easy, and use them to rehearse. Eat what you'll eat on race day, wear what you'll wear and walk the climbs as you would in the race. Time your biggest weekend for about four to five weeks out.
A shorter ultra makes a good long run, as long as you treat it as training. Two more things are worth practising: walking briskly, because you'll do hours of it, and running in the dark. Do at least a few long efforts through the evening and into the night, so your head torch, your route-finding and your eating routine have all been tested before race weekend.
Do the cut-off maths
A 30 hour limit for 100 miles sounds generous. It works out at 18 minutes a mile, or about 11:11 per km, and that includes every stop. Take 12 aid station stops at 8 minutes each and you've spent over an hour and a half standing still, which brings your moving pace down to about 17 minutes a mile.
Put your own race into the ultra pace calculator to see what you need and how much a longer stop costs. Then look up the intermediate cut-offs, which are the ones that catch people out. Start steady, because the early miles feel easy and time banked by running too hard is usually paid back later. Aim to leave every aid station with a margin, not just to reach it.
Eating and drinking
Stomach trouble ends plenty of first 100s. Eat from early on, eat little and often, and practise in training with the foods on offer at your race. Have a backup for when sweet things stop working: many runners switch to savoury food, soup or anything warm overnight. If you feel flat, try eating and slowing down for 20 minutes before you make any decision about dropping out.
Crew, pacers and drop bags
Read your race's rules closely, because they differ. On the South Downs Way 100, crew can only meet you at named locations, you get one crew vehicle and you can have a pacer on foot from mile 50, with only one running alongside you at a time. Breaking those rules can bring a time penalty or disqualification. That race also lets you send separate labelled drop bags to named aid stations.
A good pacer's job is simple: keep you moving, remind you to eat and stay cheerful at 4am. Brief your crew with a sheet of expected arrival times at each checkpoint, based on your goal and a slower backup plan. Use splits from past results for runners of a similar speed if the race publishes them.
Kit
Mandatory kit is checked, and missing items cost time. A typical list, taken from the South Downs Way 100, is a charged phone, at least a litre of fluid capacity, a survival blanket, a waterproof jacket with sealed seams, a whistle, a head torch and backup light, a spare warm layer kept dry for emergencies, and a cup. Other races will vary, so read yours and test everything on a long run. Pack spare socks and something warmer for the night, even in June. If you've run a first ultra, you'll know how quickly the temperature drops when you slow to a walk.
The taper and race day
Ease down over the last two to four weeks, dropping to around half to 70% of your peak volume, and don't try to prove your fitness with one last hard session. It can't help you by then, and it can leave you tired on the start line.
Expect at least one bad patch, often in the second half. It usually passes. Break the race into sections, think only about the next aid station and remember that dawn tends to bring a lift. Unless you're in danger or have been timed out, sit down, eat something warm and give it 20 minutes before you hand in your number.
Not sure you're ready for 100? A 50K ultra or a backyard ultra are good ways to build up, and the backyard format lets you stop when you've had enough.
