With weather forecasters predicting the temperature reaching 23C on Sunday, runners of all levels are in for a tough race.

"The body loses fluid in two main ways - sweating and breathing," says Charlie Pedlar, a physiologist at the English Institute of Sport

"An above average runner doing a three-hour marathon could expect to lose 3-5 kilograms of weight. For every kilogram you lose, you need to drink 1.5 litres of water to replace it."

Drinking that amount is impossible during the race but at least runners can keep weight loss to a minimum by taking on fluids at every water station.