Sir Ranulph Fiennes’ coach has revealed the 71-year-old explorer is overtaking people a quarter of his age during the Marathon des Sables.
Sir Ranulph is facing temperatures of more than 50C (122F) and endless sand dunes in the challenge, dubbed the toughest footrace on Earth.
He hopes to raise £2.5 million for UK charity Marie Curie by taking part.
His coach Rory Coleman told BBC Breakfast: “He is overtaking lots of people who are a quarter of his age, not just half his age. It’s remarkable he’s doing this at 71.”
But Sir Ranulph said he was trying to “subdue” his competitive side to ensure he had enough energy to finish the race.
He said: “I rely on Rory, uncle Rory, to get it right and make sure I don’t go too fast, which is a competitional [sic] element you’ve got to subdue, because it gets worse and worse and worse.
“Yesterday I thought was very difficult, but it’s nothing compared with the next day, and the next day and the next day, so you’ve got to stop any form of competition if you’re going to compete at a certain age.”
The explorer, who turned 71 in March and is battling diabetes, said enduring blisters and the heat was the toughest part of the challenge.
He said: “I’m OK in the cold but I don’t like the hot very much… I’m also finding that the blister situation is new, you’ve got to keep plastering stuff on, be sure of no sunburn.
“I really need to complete it because we are desperate to raise lots of money for Marie Curie, and without completion of course you don’t raise so much money.”
The Marathon des Sables in Morocco covers 251 kilometres (155 miles) including the distance of a marathon a day and a double marathon one day, with runners carrying everything they need on their backs.
Sir Ranulph received a message from the Prince of Wales wishing him the very best of luck before taking part in the race.
He hopes the money raised will help Marie Curie provide more free care and support to people living with a terminal illness.