A 69-year-old man from Kent is taking part in a 60-hour, 135-mile race through the Nevada desert.
Jack Denness started marathon running after he quit smoking
Jack Denness, from Rochester, starts the Badwater Ultramarathon, said to be the toughest athletic event on Earth, on Monday for the 11th time.
Mr Denness has raised over £100,000 for charities since he took part in his first desert marathon in 1991.
The part-time school caretaker will be one of 100 racing in temperatures of up to 54C (130F) in the shade.
The marathon runners have to be invited to partake in the race.
Three mountain ranges
They each have a crew on the non-stop run which provides drinks, food and foot care along the Californian route from Death Valley to Whitney Portals.
Jack's wife Mags Denness is in his crew along with four others who are from America.
The runner, who trains by taking his dog out in his home town, will take an average 250,000 steps on the race.
The money he raises will go to The Learning Centre, in Cliffe Woods, Kent, a specialist centre providing treatment for children with cerebral palsy.
Jack, who took up running after he gave up smoking 21 years ago, will finish the race with his traditional can of stout.