By ANNA JOHNSON, Associated Press Writer Tue Feb 20, 3:35 PM ET

IN THE WESTERN DESERT, Egypt - Three ultra-endurance athletes have just done something most would consider insane: They ran the equivalent of two marathons a day for 111 days to become the first modern runners to cross the Sahara Desert's grueling 4,000 miles.

"It will take time to sink in ... but this is an absolutely once in a life time thing. They say ignorance is bliss, and now that I know how hard this is, I would never consider crossing the Sahara on foot again," said American runner Charlie Engle, 44, hours after he and the others completed the run at Egypt's Red Sea.

Engle said he, Canadian Ray Zahab, 38, and Kevin Lin, 30, of Taiwan, ran the final stretch of their journey that took them through the Giza pyramids and Cairo to the mouth of Suez Canal on four hours of sleep. Once they hit the Red Sea, they put their hands in the water to signify crossing the finish line.

"We touched the water in Senegal at the beginning, and we touched the water in the Red Sea at the end. They were the bookends of our journey," Engle, of Greensboro, North Carolina, said on the telephone from a hotel room in Cairo.

In less than four months, they have run across the world's largest desert, through six countries — Senegal, Mauritania, Mali, Niger, Libya and finally Egypt.

A film crew followed them, chronicling the desert journey for actor Matt Damon's production company, LivePlanet. Damon plans to narrate the "Running the Sahara" documentary.

The trek is one of extremes. The relentless sun can push temperatures above 100 degrees Fahrenheit during the day, but at night it sometimes dips below freezing. Strong winds can abruptly send sand swooping in every direction, making it difficult to see and breathe.

Running through turbulent conditions is nothing new for these athletes who have traveled the world competing in adventure races. But they say nothing has tested their physical and mental limitations like the Sahara.

Throughout the run, the runners have been stricken with tendinitis, severe diarrhea, cramping and knee injuries all while running through the intense heat and wind — often without a paved road in sight.

"This has been a life changing event," Engle said.

The runners say they undertook the challenge to see if they could accomplish something that many have called impossible. They use GPS devices to track their route and teamed up with local experts and a host of sports professionals who also followed them, along with the documentary crew, in four-wheel drive vehicles.

Typically, the three began each day with a 4 a.m. wake-up call. About an hour later, they started running. Around noon, they took a lunch break at a makeshift camp, devouring pasta, tuna and vegetables. A short nap on thin mattresses in a yellow-domed tent usually followed before they headed out on the second leg of their day's run.

Finally, around 9:30 p.m., they called it quits each day, returning to camp for a protein and carbohydrate-packed dinner before passing out for the night.

Despite the preparation and drive to finish, the runners said they often questioned — mostly to themselves — what they were doing. Zahab described stopping one recent day for a bathroom break only to discover the wind was blowing so harshly that he couldn't keep the sand out of his clothes. "And I thought to myself, 'What the hell am I doing?'" he said.

But Zahab kept going, as did the other two, never skipping a day. Most days the three ran a total of 44 to 50 miles — sometimes a little more, sometimes a little less.

They were interviewed by The Associated Press on Saturday — day 108 — on the side of a road about 112 miles from Cairo in Egypt's harsh Western Desert, part of the greater Sahara.

At several points in their trek, the athletes stopped near sparsely populated wells to talk with villagers and nomads about the difficulties they face finding water. That marked another goal of the run — raising awareness for the clean water nonprofit group H2O Africa.

"We have seen firsthand the need for clean water, which we take for granted in North America. It's such a foundation for any community," Zahab said during day 108's lunch break. The three plan to fund-raise for the group after they return home and finish recuperating.

"It started off as a huge motivator, especially as we passed through countries where the water wasn't clean," Engle said.

But as the trio's bodies became more depleted, the focus was "the day-to-day battle to stay alive and keep moving," he said.