It takes a certain type of person to decide to run 151 miles over six days in 50C heat, in a desert, but when Karen Gent heard about an event dubbed “the toughest race in the world” she decided it was a challenge she had to face. “I’ve always got a buzz from sport and I’d started doing triathlons,” she says. “I thought: ‘If I’m going to do anything – this is definitely the race’.”

That was two years ago. Cut to last week and Gent, 32, was beginning to realise the magnitude of the task. Fifteen miles in and things were not going to plan. Utterly exhausted by the effort of running through sand and over rocky roads, she realised her feet were black with bruises and the skin had been stripped from their soles.

“It was incredibly painful,” she says. “I’d started off wanting to do really well, but now I had to change my mindset. It wasn’t about where I finished any more; it wasn’t even about about finishing; it was about survival.”

Welcome to the Marathon des Sables (MDS), the annual desert “ultrathon” that has come to be seen since its inception in 1986 as the pinnacle of endurance running. Athletes must cross scorching salt flats, treacherous sand dunes and shadeless, dried-out watercourses, or wadis. It’s not as though they are running light, either.

Loaded like pack horses, they carry food and water, stoves, sleeping bags and clothes. Tents are provided by the organisers at designated stopovers. The route through the Sahara changes each year and is not revealed to participants until the day before the race, so none of the contestants knows what lies ahead.

The risks are very real. In 1994 Mauro Prosperi, a police officer from Rome, lost the trail in a sandstorm. He wandered through the desert for nine days and was forced to drink his own urine before he was found by a nomadic family and taken to hospital suffering from dehydration. Last year another competitor wasn’t so lucky: Bernard Julé, a 49-year-old Frenchman, died of cardiac arrest during the race.

The event seems to have a magnetic pull over those runners for whom more conventional marathons have lost their lustre. As is attested by the recent boom in the popularity of Ironman contests, in which men and women push themselves to ever more extreme physical limits, no matter how hard a race, there will always be someone ready to take it on. From the 135-mile Badwater Ultramarathon, through Death Valley, in California, to the Yukon Arctic Ultra, across 100 miles of frozen wasteland, no place on earth is deemed too inhospital to attract a race.

Even among these extreme events, however, the MDS stands out as the toughest – and one of the most popular. Over the past 22 years about 10,000 people have attempted to complete it, and each year it has grown: in its first year 23 people took part; in this year’s event, which finished last week, there were 801, of whom 254 were British (altogether, 32 nations were represented). That suggests we have more than our fair share of individuals determined to push themselves to the limit and beyond. It’s not even as though it’s cheap. Just taking part costs about £2,000 and the race has a two-year waiting list – the 2010 MDS is already fully booked.

So how to you prepare for a race such as this? Gent, who recruits for a bank in the City, began by running into work from her home in Clapham, south London, every day – about 6½ miles. “My base fitness was pretty good because of all the triathlons I’d been doing,” she says. “I ran with a 6kg pack. Then in October I started running home from work as well. I’d run in, shower, do my day job, then run home again, five days a week. It was like the film Groundhog Day. You have to be incredibly disciplined.”

At weekends she ran even further with her pack but says it still wasn’t enough to prepare her for the MDS. “If truth be told, I did all the wrong training. For a start, it was so hot in Morocco. The people who can train in similar climates tend to fare very well – the race was won by a Moroccan, and the Jordanians did very well too. And the pack was so much heavier than I was used to – about 10½kg with all the water we carried – so what I should have done at weekends was start with a 10kg backpack and just walk for seven hours nonstop.”

The state of her feet at the end of the first day meant that Gent had no hope of running the remaining 130-odd miles.

Instead she paired up with a fellow Englishwoman and decided to walk the rest of the event. Having a wingwoman was priceless, according to Gent: “If I hadn’t paired up with Sarah I wouldn’t have got through the week, whether I was running it or not. There were days when you were walking across salt flats and could not see the end – it was soul-destroying.

“On the longest day, we had to walk more than 40 miles. In the afternoon we walked through a wadi that seemed as though it would never, ever, end. The ground is incredibly dry, the heat is unbelievable, there’s no shade and the sand underfoot makes you feel like you’re walking backwards. What you really want to do is sit down and say: ‘I’m sorry, I can’t go on’, but I knew if I slowed down I’d slow Sarah down. We talked about anything to keep us going, from US politics to the Spice Girls tour. Sometimes we walked in the dark, and it was nice to have someone beside you then – it gets dark very suddenly in the desert.”

There are some hazards, though, for which no amount of camaraderie can prepare you – such as a sandstorm. “You feel unbelievably helpless because you can’t get away from it,” Gent says. “It was about 1am the night before the longest day, so we wanted to get a decent night’s sleep, but the wind was raging outside, and the tents were full of sand.

“I hid completely in my sleeping bag with the strings tied tightly around my face and goggles on to protect my eyes. You literally crunch. There’s sand in your mouth, in your hair; it’s hideous. We got three hours’ sleep that night.” So was it worth it to finish a lowly 728th? “Yes, phenomenal,” Gent says, now back in London but without several toenails, which have fallen off since the race. “Crossing the finish line was incredible – relief; joy. Sarah and I plan to meet up in a few weeks – it will be nice to enjoy a glass of wine together and eat off a plate with a knife and fork, instead of eating salt tablets and freeze-dried food out of a bag.

“My boyfriend has been telling me to promise never to do it again – I think he had a more emotional week than I did. And, no, I’d never do it again. But one night I was talking to someone who’d swum the Channel and it sowed a seed. I might do that next.” Let’s hope the chilly British seas are more forgiving.

You can follow the progress of Gent and other British competitors in a documentary to be shown on May 13 on ITV4

Epic figures

151 Total length in miles, split into six daily sections of 15, 21, 24, 51, 26 and 14 miles
5,000 Prize money in euros for winner
2 Number of times this year’s victor, Muhammad Ahansal of Morocco, has now triumphed
10,000 Number of people to have run the marathon since it started in 1986
23 Runners in the inaugural race
32 Nations represented this year
254 Number of Britons in the 2008 race
2.6 Miles of plasters handed out
97 Litres of disinfectant used
3,700 Painkillers taken

On your back

— Food: enough for 2,000 calories per day. Freeze-dried cereal, Peperami, dry-roasted peanuts, pistachio nuts, Haribo sweets, Jelly Tots, freeze-dried ready meals such as chicken korma and sweet-and-sour chicken
— A stove for heating water
— Two 1.5litre bottles of water
— Sleeping bag
— Wet wipes
— Mandatory kit: venom pump for snake bites, torch with spare batteries, 10 safety pins, compass, whistle, knife, tropical disinfectant, signalling mirror, aluminium survival sheet, distress flare, salt tablets