NEW YORK (FORTUNE Small Business Magazine) - David Kuhnau was 75 miles into a 100-mile leg of a race in Antarctica when the blizzard struck. His lungs were already burning, his feet were covered with open blisters inside his trail shoes, and within seconds he couldn't see anything through his expedition goggles except slanting lines of snow.

Through semidarkness, he shouted to his two teammates that he didn't think he could make it. One yelled back, "The hell you're not finishing!" Four hours later, with snow still coming down, Kuhnau and his team finally stumbled across the finish line. It was about 5 A.M., and the 100-mile course had taken them 25 hours and 29 minutes.

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RESOURCES The 4 Desert Series starts again this month with a race in the Gobi desert (racingtheplanet.com). A few other entrepreneurially run options for the most hard-core endurance athletes: * The Kiehl's Badwater Ultramarathon is a two-day, 135-mile run starting in Death Valley and climbing 13,000 feet across three mountain ranges. July 24 to 26; badwater.com. * Race Across America, named the toughest endurance event in the world by Outside, is a 3,000-mile, coast-to-coast bike race. Competitors ride as much as 20 hours each day. June 11 to 20; raceacrossamerica.org. * The Lean Horse Ultra spans 100 miles in Hot Springs, S.D., and offers slightly less insane 50-mile and 50-kilometer options. Aug. 26; leanhorse.com. Research provided by Chuck Marvin, FSB Magazine.

Photo gallery: See the Antarctica race.

Kuhnau, 36, who owns a software consulting business in Chicago, competed in January in a five-day, four-stage race spanning some 150 miles in Antarctica and known as the Last Desert competition. The race was organized by a for-profit venture called RacingThePlanet, which coordinated similar ultramarathons throughout 2004 and 2005 as part of its signature 4 Deserts series, featuring races in Chile's Atacama Desert, the Gobi in China, and the Sahara in Egypt.

Only the 18 runners who completed those three qualified for the Antarctica race, and 15 of them, including Kuhnau and his two teammates, took on the challenge. (The series begins again this month, with the Gobi race on May 28.)

Part spiritual quest, part kamikaze mission, 4 Deserts has become the next big endeavor for people who have raced every marathon, Ironman triathlon, and Eco-Challenge out there. Before the desert events, Kuhnau had completed 17 marathons, three Ironman competitions, and, as he says, "too many triathlons to count."

While most racers prefer to go solo, he competed as part of a team of three (all entrepreneurs), because they wanted the company. In 2004, Joel Burrows, now 30, and Nancy Fudacz-Burrows, 38, a married couple who own a gym in Chicago called First Step Fitness, traveled with Kuhnau to RacingThePlanet's trek in Atacama, Chile.

The course there offered almost no shade and traversed salt flats with daytime temperatures near 120 degrees, even in midwinter. In that race, Kuhnau lost a big toenail, shed 17 pounds, and contracted bronchitis and strep throat. "I wanted to die," he says.

The next spring the team flew to China to run in the Gobi desert, which competitors refer to as the "oven" because of its relentless sun and seemingly endless expanses of bare rock. Kuhnau suffered chronic nausea and hallucinated from lack of sleep.

In September in the Sahara he lost another toenail and started vomiting so violently from the 130-degree heat that he had to crawl under a Volkswagen-sized rock to cool down. But the intensity of the experiences started to become addictive. "You learn to overcome fear," Kuhnau says. "You have to if you want to get through."

Training for the races took up about 18 hours a week and was tough to schedule, given Kuhnau's responsibilities at his burgeoning company. GDI Solutions, which he launched in 2003 after working for Andersen Consulting (Research) and PeopleSoft, helps companies adopt new technology. During busy stretches of the year he manages as many as nine employees and a client list that includes HSBC (Research), Pfizer (Research), 3M (Research), and Waste Management (Research). Revenues for 2005 were about $500,000.

Before work Kuhnau would wake at 5 A.M. to get in a 30-minute bike ride or a five-mile run. Other mornings he took his work to the Lake Michigan shore, ran an eight-mile loop along the waterfront, returned to his car for an hour or so of conference calls with clients, and then put in another eight-mile loop. At night he went to the gym for weight training, swimming, and treadmill work.

"For a few months I didn't have much of a social life," he says. (One less rigorous part of his training: Kuhnau watched March of the Penguins before the Antarctica race.)

During the Atacama, Gobi, and Sahara competitions he also brought work with him. After stages he went straight to a laptop and answered e-mails, and in the middle of Atacama he was able to troubleshoot technical issues for clients on his PDA. (Surprisingly, he found decent cell service.) "I was in the middle of nowhere, e-mailing people," he says. The long-distance charges hurt, though: "When I got the bill, it was something like $1,000."

Life altering is the first expression most use when describing the desert races. According to Mary Gadams, 41, the U.S.-born founder and CEO of RacingThePlanet, the desert-based ultramarathons are so transforming that after completing one, about a third of the competitors quit their jobs to do something more fulfilling.

A few have launched companies. Kuhnau and his teammates are keeping their current businesses but also are opening an adventure-clothing firm, FunkyMonkeyWear, and a motivational-speaking business, Bluefire Media, to talk to corporate groups about their experiences and teamwork in the desert races.

A veteran marathoner and ultramarathoner, Gadams organized the first 4 Deserts race in 2003, after holding executive-level jobs with Samsung and Wachovia ("I knew I didn't have the passion for the corporate world," she says), and her company turned profitable in 2005. About 100 competitors enter the races, 80% of them finish, and half of those return for a second race.

The majority are in their mid-30s and early 40s, about 20% are women, and they come from all over the world. Many are executives or own a successful business, which allows them to afford RacingThePlanet's fees ($2,600 for each of the Atacama, Gobi, and Sahara races and $9,500 for Antarctica; Gadams planned small increases for 2006). In addition, Kuhnau and his team spent about $10,000 on equipment, including $145 polar expedition goggles and $280 snowshoes.

When Kuhnau left Chicago for Antarctica in late January, he carried with him the ashes of his sister, who had died of cancer in December. "She was someone who pushed the envelope in life and wasn't afraid to try something once," he said. "I brought part of her ashes with me as a way for me to show her a part of the world that she would have loved to have seen."

Throughout the five-day, 150-mile race, wind-chill temperatures dropped as low as 30 degrees below zero, capable of turning spit to ice in midair. Because it was high summer, daylight stretched for 20 hours, but snow and whiteout conditions were expected.

Kuhnau had told his clients that they wouldn't hear from him--he was too far off the grid. "I knew it was going to be the most dangerous of the races, especially because of the cold," he says. "If you did something stupid out there, the consequences would be big."

The first day, all 15 competitors ventured on a 6.2-mile test run. Kuhnau wore three layers of clothing, and his ten-pound backpack was stuffed with a medical kit, 2½ liters of water, Swedish fish candy, and extra shirts, hats, gloves, and SmartWool socks. He had plenty of pills too, including salt tablets to prevent dehydration, antibiotics to relieve bronchitis, and Advil to ease his aching muscles.

The second and longest stage started on the Antarctic Peninsula in an area known as Hope Bay. Halfway through that 100-mile run, Kuhnau paused on a jagged peak overlooking an ice-covered bay where icebergs loomed, and scattered his sister's ashes.

The blizzard hit shortly after, obliterating the course. Kuhnau and his team managed to avoid getting lost. (He says they did lose their way during all three of the earlier desert races; in Antarctica, Gadams says, Arctic birds stole some of the marking flags.) More than a day later the team finally finished. That made 106 miles completed, but the racers weren't done.

The next day they ran a marathon on the gloom-inspiring Deception Island, another of the Shetlands and a 12-hour boat ride away. A collapsed volcano that once hosted a Norwegian whaling station, the island presented competitors with 26.2 miles of mountainous terrain, thermal baths, and a graveyard of rusting buildings and decrepit machines formerly used to boil whale blubber.

Chinstrap penguins abounded. Much of the footing was loose volcanic cinder-ash, which Kuhnau says made the running at times feel like trudging through fresh snow. But it was Kuhnau's and his team's best day; they finished the course in five hours, 44 minutes, putting them in sixth overall.

The final 18-mile leg was back on King George Island. In the sun the glacier was melting and the course slippery with slush ankle-deep in places. Everyone was exhausted, all bodies on tilt. For most of the race, Kuhnau's feet were soaked, and he was so tired that he was hearing voices.

"My wheels just came off," he says. After two hours and 53 minutes, the Last Desert was finally over, with Kuhnau and his team in sixth place. Scott Smith, 49, an ROTC instructor from Bloomfield, N.M., took first, followed by Chuck Walker, 39, a deputy sheriff from Stockton, Calif.

When the competitors departed the next day, Kuhnau was proud of the accomplishment but also somewhat dispirited. He was headed for some sun and downtime in Buenos Aires, but the biggest race of his life was over. "It was sad," he said. "It's the end of a journey. The last chapter of a four-chapter book, and now it's over and you're asking, What now?"

Gallery: See the Antarctica race.

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