Two fundraisers are attempting to run the South Downs Way in less than 24-hours - the equivalent of four non-stop marathons.
The challenge has been attempted but abandoned before
Rob Ravilious and Gordon Elliot set off on the 106-mile (171km) run from King Alfred's statue in Winchester, Hampshire, at 0700 BST on Saturday.
They hope to reach the post at the end of the way on Eastbourne seafront, East Sussex, by the same time on Sunday.
The challenge is in aid of Naomi House Children's Hospice in Winchester.
Mr Ravilious, from Guildford, Surrey, and Mr Elliot, from Sawbridgeworth, Hertfordshire, will run unsupported, carrying their own clothes, water and food - including energy bars and pepperoni sticks.
They have attempted to run the South Downs Way non-stop before but were forced to stop after 70 miles because of exhaustion and bad weather.
The pair practised for their latest feat at this year's 20th anniversary Marathon des Sables in the African desert.
The Marathon des Sables is 150 miles (240km) long and took seven days to complete in temperatures of more than 50C.
Mr Ravilious told BBC News: "No matter who you are you'll have a low point. I am expecting that to be about 4am, when the body's blood pressure is lowest.
"We'll get through it if we can both pick each other up out of the low points."
Naomi House provides respite and terminal care for children and supports their families, relying on voluntary donations to meet annual running costs of £2.5m.