ALBUQUERQUE, N.M. — Ultra-marathon runner Micah True died of natural causes while on a routine 12-mile run in late March in the mountains of southwestern New Mexico, authorities said today.

An autopsy showed that True, 58, had cardiomyopathy, or heart disease, and there was some enlargement of his heart, which can cause an irregular heartbeat during exertion.

True’s body was discovered on March 31 along a stream in a remote part of the Gila Wilderness. The search for him began days earlier after he failed to return from a run.

Friends theorized that he stopped at the stream to wash up after a fall while running through the rugged terrain.

Remembered as a legend and an inspiration among runners, True, nicknamed “Caballo Blanco,” was known for his big smile and infectious love of running.

He had been involved in ultra-marathons for years, but it wasn’t until he became friends with the indigenous Tarahumara of northern Mexico that the direction of his life came into sharp focus. The Tarahumara are known for their extreme running prowess.

True founded the 50-mile-plus Copper Canyon race and directed it for the last several years. This year had marked a record turnout for the grueling event, which sends participants, many wearing only sandals made of discarded tires, plunging into deep canyons and across mountains and rivers.

True would spend much of the year living among the Tarahumara, or Raramuri, as they are also known.

It was in the canyons where True, a struggling runner himself, got rid of his running shoes, put on a pair of sandals and learned to run the way the Tarahumara do — easy, light and smooth.

Friends said True was healthy, so his death came as a surprise.

“This is a guy who could set out with a little bag of ground corn, a bottle of water in his hand and be gone all day. The day before he died, he did a six-hour run,” said Chris McDougall, a friend of True and author of “Born to Run.”

The book chronicles True’s efforts to develop the Copper Canyon race and draw international attention to the Tarahumara way of life and their love of running.