If your last sporting feat was to run 50 marathons in 50 days, how could you take it to the next level? Dean “Ultramarathon Man” Karnazes came up with this: run 1,000km (621 miles) across four inhospitable deserts, through temperatures as high as 40C and as low as -40C, and end up in a blizzard in Antarctica.

It would be the ultimate ultramarathon, and even for Karnazes - an athlete who has been held up as a perfect specimen of physical fitness and examined by doctors in the interests of medical science - it was almost too much. “I’ve never known a challenge like it,” admitted the 46-year-old American as he boarded a boat for home last week in a snowstorm.

Last March, Karnazes was one of a handful of runners hoping to be first to complete the Four Desert Challenge in one year. The four week-long ultramarathons that make up the event take competitors through the planet’s most hostile landscapes. This year’s challenge started in March in the Atacama desert, in Chile, where there is no record of rain ever falling in one of the stage locations. It continued with the Gobi Desert March (dubbed the Death March by competitors) in China in June, moved to the Sahara in Egypt in October and finished in Antarctica last weekend.

During each race, Karnazes had to fend off the usual aches and pains of the long-distance runner, plus heat exhaustion, dehydration, dizzy spells, hallucinations and, in Antarctica, the ever-present risk of frostbite. He had to cart 20lb of supplies on his back, including food, water, camping gear and a knife for dealing with the inevitable blisters. Some of the stages are 50 miles long, and Karnazes needs access to plenty of calories without overloading his rucksack. That means an unappetising week of dry granola for breakfast, energy bars for lunch and beef jerky for dinner. “You take enough to sustain you and no more,” he says.

Frightening hallucinations have plagued him as he has succumbed to dehydration and sleep deprivation. “I’ve seen dinosaurs, tiny creatures in the sand that morph into giant monsters, and mythical characters from a book I read as a child,” he says. “Your mind plays tricks. I’ve sometimes seen snake eyes looming up towards me, and they turn out to be a pair of car headlights. It must be like LSD or something.”

Most people can stomach this kind of torture only once or twice in a lifetime (most competitors sign up for just one race), which is why no one has managed to complete all four desert races in a year. At the Antarctic stage, only Karnazes and Paul Liebenberg - a doctor from the Australian outback - were still well positioned to achieve that goal.

Karnazes, who lives in San Francisco with his wife and two children when he’s not off torturing himself around the world, may be approaching 50 but he shows no sign of quitting. He has a resting heart rate of just 39bpm, which rises to just 150bpm when he’s running, and he’s been nicknamed “the Terminator” by the other competitors. You’re not sure whether he’s man or machine, but he always runs you down, they say.

Having won the first race in the Atacama by 35 minutes, Karnazes finished fourth in the Gobi, second in the Sahara and went into the Antarctic race ahead of Liebenberg, who is 13 years his junior. The final race took things to another level. After running across the hot, dry Saharansand for race three in October, Karnazes had just a few weeks to prepare for running in subzero temperatures on ice.“I trained by running in deep sand on the beach near my home while wearing a thermal suit,” he says.

“The real challenge is learning how to run in thick clothing. If I feared anything, it was the air freezing my trachea [windpipe]. I had to wear a special neoprene mask to prevent that. I also had a pair of Gore-Tex shoes, which were one size too big. This let me put heated pads above and below my toes to prevent frostbite.”

He also had waterproof socks, goggles to protect his eyes, a fleece, leggings, a jacket, windproof gloves and, underneath it all, a thin base layer of clothing made of a material designed to wick moisture away from the skin, most of it provided by the North Face, his sponsor. Before he could get to the start line in Dorian Bay, on the west coast of Antarctica, Karnazes had to endure a gruelling sea crossing from Argentina in choppy waters. Two days into the six-day race, having won the first stage, he was overcome by dizziness, brought on by a bad reaction to the seasickness tablets he’d taken on the boat, and had to battle dizzy spells for two days. Then bad weather began to close in.

By the morning of the fifth day, the competitors were in the middle of a snowstorm and the final two days of the race had to be abandonded. “Stages five and six would have been impossible because of the storm. It was a shame. I was looking forward to stage five, the longest one, to really push myself to the limit.”

He was a few kilometres short of the 1,000km total but Karnazes was declared the overall winner of the challenge, and, along with Liebenberg, a few minutes behind, the first person to make it to the end. “I run from the heart - that’s what gives me longevity,” Karnazes says. “I feel the heat and cold like everybody else, but long-distance running is as much a mental as a physical challenge. I know how to conquer my demons, but four big events like this takes a massive toll on your body. Now I’m ready to put my feet up, though I expect I’ll rest for a couple of days, then see my running shoes and head out of the door.”

RACE STATISTICS

250kmDistance of each race, run over six days
2The number of competitors, including Dean Karnazes, who completed all four races
5,000The number of calories Karnazes consumed every day
20lbThe weight of his rucksack
5lbAmount of body weight Karnazes lost per race
38Average age of the competitors. Karnazes is 46
24hr, 38minThe fastest time, set by Ryan Sandes, 26, from South Africa, for the Gobi desert race this year
83hr, 1minThe slowest time, set in the Sahara by Monica Otero, 51, from Brazil, who has survived cancer