Sir Ranulph Fiennes, who turned 71 in March, will attempt on Sunday to become the oldest Briton to complete the Marathon des Sables in aid of charity.
The veteran explorer will take part in a gruelling six-day ultra-marathon in the Moroccan desert.
He will face temperatures of more than 50C and endless sand dunes in a challenge labelled the toughest footrace on Earth.
It covers 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, who has suffered two heart attacks, undergone a double heart bypass, a cancer operation and has diabetes, is embarking on the massive challenge to raise £2.5 million for UK charity Marie Curie.
Chief executive of Marie Curie Dr Jane Collins said: "We are thrilled and privileged that Sir Ranulph has chosen to undertake another huge challenge for the charity. He has quite literally gone to the end of the earth and back to raise money for Marie Curie.
"His determination and ability to push himself to his limits is truly inspiring.
"We hope Sir Ranulph will inspire others to take on their own personal challenge in aid of Marie Curie and help us care for more people living with a terminal illness."