Video report by ITV Calendar's Adam Fowler
The Montane Spine Race is one of the most gruelling endurance challenges in the world - competitors have a maximum of seven days to run 268 miles.
Among them will be 54-year-old Gary Chapman from Haworth, and it'll be the seventh time he's done it.
"The last time I did it I had a total of six hours sleep in four and a half days that I took," he said. "Previously, I've taken as much as eleven hours sleep, which was, I guess, a luxury."
Runners set off from the village of Edale in the Peak District and go all the way to the Scottish village of Kirk Yetholm.
Most of Gary's time will be spent running in the dark, often in the pitch black and the organisers schedule it for this time of year and the organisers schedule it for this time of year so the runners can experience the full intensity of the British winter.
They're warned to expect deep snow, ice, mud, bogs, ground water, storm force winds and driving rain.
"It just challenges everything your decision making, your physical preparation, the mental side is enormous, and indeed it's actually more important than the physical side in many respects because you go through so many low periods you want to give up, you're in, you're so fatigued," he said.
"I've fallen asleep on my feet before and just ended up having to throw myself into a bivvy bag on the Pennine Way just to try to get some energy back into my body so I could get to the next place of safety."
Gary's run two and a half thousand miles in the last year.
Gary's been sleeping in the basement, away from his wife and children, to minimise his risk of catching Covid.With the target of raising money for his local primary school he's determined nothing will stop him achieving what many of us could never dream of attempting.