How many gels? When do I take them? What if they make me feel sick? And is the wall actually avoidable or is it just part of it?

Mostly avoidable. The wall is largely a fuelling problem, and fuelling is the most fixable thing in the marathon.

Why you run out

You’ve got roughly 90 minutes to two hours of stored carbohydrate. Fat you’ve got plenty of, but you can’t burn it fast enough on its own to hold marathon pace.

So around 18 to 20 miles the tank empties and it all goes at once. Not a mental thing. Not weakness. You’ve run out of fuel.

The numbers

30 to 60 grams of carbohydrate an hour once you’re past the first half hour. Most gels are 20 to 25 grams. So two to three an hour.

For a three and a half hour marathon that’s six to eight gels. Most people carry three, take two, and wonder what happened at 21 miles.

Start early. First at about 40 minutes, then every 25 to 30. Taking one when you already feel empty is far too late. You’re then digging out of a hole while running.

The bit everyone skips

Your gut needs training the same as your legs. Six gels on race day when you’ve never taken more than one in training is asking for a very bad morning.

Practise it on every long run from twelve weeks out. Same gels, same timing, same drinks. By race day you should know it works.

If a brand disagrees with you, find out in September. Not in the last six miles of something you’ve spent four months building for.

Drink

Drink to thirst. Simplest rule and the right one for nearly everybody. Grab water at most stations, few mouthfuls, move on.

Don’t force it. Drinking far more than you need is genuinely dangerous, and it’s a more common problem in slower finishers than dehydration is.

Water with a gel rather than sports drink, unless you fancy finding out what too much sugar at once does at mile 19.

Race morning

Three hours before if you can. Something you’ve eaten before a hundred long runs. Toast, porridge, a bagel, whatever your normal is.

Nothing new. Nothing clever. Nothing somebody recommended online last week.

I’ve run a marathon off an all-you-can-eat hotel breakfast and it was fine, because it’s what I’d been doing. That’s the only test that matters.

Get it right and the last 10k stops being something you survive. It becomes where you start picking people off.

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Want fuelling practice built into the long runs rather than left to you? That’s how my marathon plans are written.