Bob Long, 70, just became the oldest person to finish what Guinness World Records calls the longest multi-horse race in the world. He also came in first.

“It’s nothing,” he said during a Facebook live stream shortly after crossing the finish line of the Mongol Derby last Thursday. “You just ride 650 miles on a death march.”

The annual race, a grueling 1,000-kilometer competition across the steppes of Mongolia held over 10 days, traces the former mail routes of Genghis Khan with riders changing horses every 40 kilometers, according to the website for the Adventurists, the travel organization behind the contest and other real-world obstacle courses. (Its site says it’s “fighting to make the world less boring.”)

Lara Prior-Palmer, who in 2013 became the youngest person — and first woman — to win the race, at 19, described the event in her recent memoir “Rough Magic“ as the “Tour de France on unknown bicycles.”

Mr. Long, an amateur rider from Boise, Idaho, beat out 43 competitors this year, riding about 100 hours in about seven and a half days, on 28 different horses, by his tally.

At the outset, Mr. Long was not a favorite to win, said Tom Morgan, this year’s race director and the creator of the competition for the Adventurists.

Each year, Mr. Morgan said, he spends months plotting the route, enlisting 300 to 400 Mongolian hunting families to work the race and about 1,500 horses to carry contestants across high passes, huge valleys, wooded hills, river crossings, wetland, dunes and open steppe in extreme temperatures.

It costs about $13,700 to enter, which includes a custom-made saddle, access to a medical response team and veterinarians to care for the horses, among other expenses.

“We’ve had older riders before and they’ve struggled for various reasons,” Mr. Morgan said in an interview on Tuesday. This year, participants’ injuries included a broken collarbone, a punctured lung and a broken rib. (There were zero horse injuries.)

“The fundamental essence of an adventure has to incorporate the unknown,” Mr. Morgan said. “The most rich experiences come from this sort of slightly chaotic moments when you’re not completely prepared.”

But Mr. Long is now “a legend,” he said.

On Tuesday, Mr. Long had just returned to Boise after several long-haul flights from Ulaanbaatar, Mongolia. Reached by telephone, he said that he was getting used to sleeping in a bed again and that his gastrointestinal system was reacquainting itself to American food.

Mr. Long, who retired as the chief technology officer for a health care technology company, said he was inspired to enter the competition after watching “All the Wild Horses,” a film about the race.

“It took me about 15 minutes to decide that I could do that,” he said. He “hated” to think that he couldn’t.

Mr. Long had grown up riding in Wyoming, which he said had similar open landscape to the steppes of Mongolia, and he prepared physically and psychologically for the race.

“In January I couldn’t spell endurance,” he joked.

Over the course of several months, he trained with previous winners of the race, practicing riding four or five horses a day as far as each could go, and learned how to change horses efficiently.

It was this training that Mr. Long said put him in good stead for the race, even against younger competitors.

“Preparation trumps youth,” he said.

Each time he was ahead during the race itself, he gave his horse a blue ribbon that he’d brought from home. He handed out flavored lip balm, colorful stickers and barrettes to the children in the homes where he stayed throughout the race. And he made little baggies with a pocketknife and two cigarettes to give to each of the herdsmen.

“It didn’t take long for them to stand in line to help me,” he said.

On the third day of the race, Mr. Long took a shortcut and pulled ahead of the other riders.

It was unnerving to be the leader, he said, “to be out in the middle of nowhere and not be able to know for sure if you’re making the right decision, or to confer with anyone.”

But he was familiar with the solitude of the wilderness from his childhood.

“I had opportunity to give thanks for the blessings I was experiencing and to talk to God,” he said. He also sang “There’s a Coach Comin’ In” from the 1969 film “Paint Your Wagon.”

The Mongolian herdsmen awarded him a horse, which his daughter wants to name “Derby.” But he was not sure how to bring it back. So he said he left the herdsmen enough money to take care of it for a year.

Mr. Long said that he was thinking about reaching out to Defense Secretary Mark T. Esper, who received a horse from his Mongolian counterpart earlier this year, to see if he might give his horse a lift home.

During his race, Mr. Long wore an angel pin and carried a photograph his mother had taken of him as a boy with his father and his Shetland pony “Buttons.”

Next week, he will ride in Wyoming to scout a location for his parents’ ashes, which he’s placed in bronze boots, so they can see the Tetons, and be close to his brother, who was killed in an airplane crash.

“I believe my mom was riding with me,” he said. “I think she helped pick me up a couple of times.”