Micah True, a one-time prizefighter who adopted a nomadic lifestyle and became a captivating figure in the world of ultramarathon running, was found dead March 31. He was 58.

On March 27, Mr. True went for a 12-mile run in the Gila National Forest, in southwestern New Mexico, where he had stopped on a road trip to Phoenix.

When he failed to return after a few hours, search parties began to scout the area. His body was discovered in a ravine four days later. An official for the New Mexico medical investigator’s office said a determination of the cause of death was pending test results.

Mr. True’s remarkable endurance and effortless running style earned him an international reputation as one the top competitors in ultramarathons, in which racers run distances of 50 miles or more.

He was prominently featured in the Christopher McDougall’s best-selling 2009 book about ultramarathon racers, “Born to Run: a Hidden Tribe, Superathletes, and the Greatest Race the World Has Never Seen.”

“He came across as a real life gunslinger,” McDougall said in an interview. “Now, real-life cowboys aren’t cuddly. But when you got him out for a run, his entire personality transformed. He ran with a smile on his face.”

In the remote desert canyons in northwestern Mexico, where he lived for many years in an adobe hut he built by hand, Mr. True logged more than 5,000 miles a year running on the steep, rocky trails.

Earlier in his life, he had thrived in underground boxing rings in Colorado, where he fought for money in fixed matches billed as the “Gypsy Cowboy.”

His body battered, he moved to Hawaii and experienced a spiritual transformation in the jungles of Maui, where he became a cave dweller near the sacred Hana shrines. After a failed relationship with a woman he met in Hawaii, Mr. True became despondent and took up long-distance running.

He traveled around “like a surfing bum or a climbing bum,” he told Boulder Weekly in 2000, “but I was a trailrunning bum.”

His journey took him to Guatemala, where he ran along the banks of the volcanic crater lake of Atitlan, surviving off the tortillas and bananas he bought in villages. Such a strange sight was the gaunt, pale figure trotting town to town that the locals named him “El Caballo Blanco,” or the white horse.

At the 1993 Leadville 100, a 100-mile trail race through the Colorado Rockies, Mr. True met a small group of runners wearing sandals fashioned out of old car tires. He learned they were Raramuri, an elusive tribal people indigenous to Mexico’s Copper Canyon region amid the Sierra Madre.

One of the Raramuri runners, the 55-year-old Victoriano Churro, won the race and finished 40 minutes ahead of the field. Mr. True finished 28th.

Hampered by injuries in the past, he was impressed by the smooth running style of the Raramuri, whose name translates as “the lightfooted ones.”

He set out for Urique, Mexico, to learn the secret of their injury-free running. Isolated from modern running shoes with spongy foam and air bubbles, the Raramuri padded around in sandals. By running on the balls of their feet, they allowed their legs to act as natural shock absorbers.

To research his book, McDougall told The Washington Post that he had traveled to Mexico to meet the Raramuri and “figure out why they were able to run these in­cred­ibly long distances deep into old age.”

McDougall learned that the tribe, also known as the Tarahumara, was not very receptive to strangers. But he heard about a tall, white “gringo” who had lived and run among them for more than 15 years.

After meeting Mr. True, he described the runner’s sinewy physique by writing: “Melt the Terminator in a cauldron of acid, and Caballo Blanco is what comes out.”

Starting in 2003, Mr. True organized the Copper Canyon Ultra Marathon to benefit the Raramuri. The annual event offered top-class distance racers the opportunity to compete against the naturally gifted runners.

The ultimate purpose of the race, Mr. True once wrote, was to come together “at the bottom of a deep canyon to share with the local people of the region, eat, laugh, dance, run, and create peace.”

The son of a Marine Corps gunnery sergeant, he was born Michael Randall Hickman on Nov. 10, 1953, in Oakland, Calif. His family moved frequently to different military bases, and the young, skinny Michael took up boxing to fend off schoolyard bullies.

He studied religion and history at Northern California’s Humboldt State University before dropping out to become a boxer full time. Survivors include two brothers and a sister.

In Hawaii, he changed his first name to Micah, after the Old Testament prophet, and his surname to True, after a loyal mongrel he kept as a pet.