Described as one of the most gruelling physical challenges in the world, the infamous Marathon des Sables isn’t the kind of event you might expect to see a man of 71 years competing in; but then again, Sir Ranulph Fiennes has made a career out of surpassing expectations.

The Marathon des Sables is scheduled to take place this April, less than a month after Fiennes reaches his 71st birthday, which means he’ll be celebrating the milestone with a 251-kilometre run over six days – sometimes in heat in excess of 50 degrees.

And while Fiennes already holds several endurance records, he’s also suffered two heart attacks resulting in a double heart bypass, on top of a cancer operation and the management of his diabetes.

For these reasons, the UK explorer has had to undergo a rigorous training regime, as well as multiple fitness tests.

“It’s going to be a challenge, but I’ve been assured my heart is mechanically up to it. The next three months are going to be full on with intense training,” Fiennes said, also noting that he hopes to raise as much as £2.5 million for the cancer support organisation Marie Curie.

Fiennes has already raised £6.3 million for the charity through several feats of endurance, including climbing the Eiger’s north face in 2007 and becoming the oldest person to climb Mount Everest in 2009, aged 65.

Cardiff-based endurance runner Rory Coleman has volunteered his time to ensure Fiennes will be in the best possible shape in April.

“It’s going to be a tough undertaking for Sir Ranulph, he’s taking on the extremes of the heat, desert storms and endless sand dunes, instead of the minus 50 degrees centigrade, high altitudes, snow and ice that he’s far more used to conquering.”

The Marathon des Sables will take place from April 5-11, 2015 and you can follow the story, as well as donate to the cause, at the Marie Curie website.

Fiennes and teammate Charlie Burton are the only people to have journeyed around the world on its polar axis using surface transportation alone and he has been described by the Guiness Book of World Records as the ‘world’s greatest living explorer’.