This year's London Marathon is likely to be the hottest yet, so how do you run a marathon in the heat?
On Sunday some 40,000 people are expected to take part in the race in temperatures that are forecast to reach up to 23C (73F), which would break the previous record of 22.2C (72F) in 1996 and 2007.
Organisers have contingency plans in place, including more water at drinks stations, more showers, and extra ice at first aid stations.
But Greg Whyte, professor of sports science at Liverpool John Moores University, says he nevertheless expects "carnage" in terms of the number of people who become ill during the race.
That is because not only is it due to be hot, but temperatures have risen very quickly, so runners will not have had time to adapt.
"Most of the runners will be completely un-acclimatised to the heat, so they're not in a good position to cope with it; it's very difficult to combat that," says Prof Whyte.
That means it is very important for runners to adapt their strategy for the race.
Reduce your pace
Many marathon runners will have a time they hope to beat when they complete the 26.2 miles on Sunday.
But experts say one of the most important measures people can take when running in the heat is to put such targets aside and make sure they run a slower marathon, starting from the beginning of the race.
It is those who rigidly stick to their pre-race plans who will be at the greatest risk of harm.
"They've set their target for their race, particularly in terms of time, and they'll shoot for that time irrespective of what else is going on," says Prof Whyte.
"And that's where the mistakes will be made.
"What you'll see with the elites is you'll know they've adapted their race strategy because there won't be records broken if it's really hot," he adds.
People can monitor their pace with a watch or even use wearable technology to check their heart rate and try to match it to levels reached during their longest runs in training.
In hot marathons, runners can lose up to four litres (seven pints) of fluid through sweating and exhalation, putting them at risk of dehydration, which is why it is important to drink enough water.
Prof Whyte says it is important to do this "little, often and early" and not wait to become dehydrated.
Dr Francois-Xavier Li, a lecturer in sports sciences at Birmingham University, says it is important to also consume carbohydrates with these liquids because they help your body to hydrate better.
This can be done with sports drinks or by having small amounts of food with drinks, including energy gels.
Some experts warn against taking on too much liquid, because in extreme cases it can lead to water intoxication, a potentially fatal condition.
Prof Whyte said this was "very uncommon" and people were far more likely to end up dehydrated.
Dousing your head and the back of the neck with water or by using the showers also helps to reduce body temperature.
But it is important not to get too wet as this raises the risk of parts of the body becoming blistered or chafed, particularly the feet.
Be prepared in advance
While experts say pacing and hydration are the most important factors for running a marathon in the heat, there are other important preparations.
Sun cream is important to help protect the skin from burning and becoming too hot, but it is important to get a cream that is water-resistant so that it does not drip into your eyes.
Clothing can also make a difference, with light-coloured and loose-fitting clothes recommended, along with a hat for protecting runners from the sun – as long as the hat is not made from thick material.
But what if you are one of those who are planning to run in a costume?
The London Marathon organisers have warned people to "think carefully whether that is still appropriate in the conditions".
But Prof Whyte thinks it would be wrong to discourage people from wearing such outfits – often for charity – and says these people just need to be extra careful to make the necessary adjustments.
Listen to your body
While steps can be taken to prevent harm, what should you do if you end up reaching "the wall" – that point where it feels too hard to carry on?
If you find yourself feeling tired, you should slow your pace or even walk, experts say.
But what are the signs that you might actually be in danger of becoming ill?
Headaches, confusion, loss of muscular control, feeling cold or no longer sweating could be signs that you are in fact sick.
If you feel any of these symptoms, you should slow down and make sure to drink.
And if this does not make you feel better you should ask for help at one of the first aid stations, rather than wait by the side of the road where it may take longer for you to receive attention.
So overall, how should you approach a marathon in the heat?
Above all else, you should should try to have fun rather than focus on race goals.
"The vast majority of runners, they're not getting paid to do this, they're not going to get any prize money, they're not going to earn their fame and fortune out of the time they run," says Prof Whyte.
"What they should be doing is enjoying it, and really judging how they are responding as to whether they're enjoying it or not, and try to bring it down to that very basic level constantly.
"If you are a first timer, make sure you've always got a smile on your face," he adds.