Racing for 150 miles in the 49C heat gave Ben Edwards the chance to raise cash and do a bit of research. Alex Klaushofer joins him

Standing in the burning Sahara, Ben Edwards refills his water bottles in the only shade there is - a couple of feet afforded by the shadow of a Land Rover.

By his own admission, he is not too good in the heat. "As soon as the sun goes up, I go all wobbly," says the 32-year-old senior lecturer in sports and exercise science at Liverpool John Moores University. But midway through the Marathon des Sables, a 150-mile race across the desert in southern Morocco, Edwards is dealing with far more than just temperatures of up to 49C. Over seven days, with 776 other runners, he must orient himself over plains, dunes and expanses of rocky desert, running the length of an ordinary marathon - 26 miles - again and again. Although the competitors receive a water allowance at checkpoints, they must carry all the food and equipment they need on their backs. At night, they share traditional Berber tents in a camp devoid of washing facilities or any creature comforts.

These combined challenges make the event as much a test of planning, mental resilience and self-knowledge as physical fitness. Over its 20-year history, this ultra-marathon has become known as the "toughest footrace in the world".

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Edwards takes more than just a passing interest in how his fellow runners cope with the enormous challenge. Collaborating with his colleague Martin Eubank, he hopes to use the race to help reveal how such events influence the emotions. He hands out forms for runners to record how their moods shift. They combine specific questions with an open-diary format, designed to elicit the kinds of adaptive mechanisms used by athletes during prolonged exercise in extreme environments. He will also lay the ground for a second research project into the effects of exertion and heat on cardiac fatigue and injury in athletes, planned for next year's Marathon des Sables.

The first two days go well for Edwards, who traverses wadi, dune and summit with tent mates George Leicester-Thackara and James Mersey-Thompson. But the long stage, a gruelling 50-mile stretch, puts him to the test. Damage to the soles of his feet suffered earlier in the race means he has to give up any idea of running and instead he walks the entire stage. With his progress reduced, he spends 22 hours on his feet, snatching just an hour's nap at the final checkpoint.

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The following day, he limps over to visit the race doctors. "I've effectively run the pads off my feet," he tells me. A sandstorm has blown up, making it difficult for the competitors to get any rest on what is designed to be a recovery day. A mucky yellow fog swirls around us. Even in the medics' tent, as Edwards tapes up his feet, the wind constantly throws grit in our faces.

Despite his injuries, Edwards still has two more days to go if he is to complete the race. But he is determined to continue: "I want to get my sponsorship. I'll get round." He is driven by more than the personal challenge or a hunger for scientific data - he is running to pay back an enormous debt.

At 15, Edwards was diagnosed with testicular cancer. His family were mourning his grandmother so he checked himself into hospital for the operation without even telling his parents what was wrong. "I just got on with it," he recalls. "I couldn't really talk to my family because they were still grieving for my gran. I told my mum I was going in for an operation on my leg."

This, together with a second scare five years later that turned out to be a benign tumour in his chest, set him on a long, self-imposed task of fundraising for cancer charities. "I once priced the cost of the treatment that I had had at ?2.5 million," he says. "I hope to raise that. Then I don't have to run marathons anymore. It's like paying my dues." Previous efforts have led him to run the London Marathon seven times, raising ?2 million. He is looking to the Marathon des Sables to add a further ?100,000 for the Association for International Cancer Research.

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Edwards acknowledges that his early brushes with death have helped him develop a phlegmatic attitude to equip him to face difficulties. "I'm able to deal with situations quite well really," he says. "Crying doesn't help."

But his observations on competitor psychology suggest that there is no ideal mindset for succeeding at an endurance race. "You normally find that elite athletes work within their capabilities, and that comes from experience," he says.

As a physiologist, Edwards is familiar with the ways in which the body protects itself, shutting down or limiting activity when the physical strain gets too much. It is an insight that helps him come to terms with the fact that, while he is normally a good runner, for the latter part of this race he has had to resort to a steady plod. He says: "It's a godsend in some respects. I can't run, because my feet won't let me."

This philosophy, combined with his commitment to fundraising, gets him through every painful step of the 12-mile final leg. "When we got to the halfway stage, I thought: 'That's ?50K raised', and at the end 'we've got the Pounds 100K raised'," he says. "It's definitely the idea of raising money that helped me to go through it with a smile on my face."

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After a total of 61 hours and six minutes on the course, he crosses the finishing line in the village of Tazzarine in 656th place. He is accompanied by the race's signature camels, who bring up the rear.

Despite the pain, Edwards has no regrets. "I realised that was all I could do. I went out to compete, and ended up enjoying the countryside."

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