Search teams intensified efforts Saturday to find renowned long-distance runner Micah True, who mysteriously vanished four days ago after heading out from a lodge for a morning run in the rugged wilderness near New Mexico's Gila National Forest.

The 58-year-old True, whose extreme-distance running prowess is detailed in the book "Born to Run," set out on what - for him - would have been a routine 12-mile run Tuesday from The Wilderness Lodge and Hot Springs, where he was staying. True, who left his dog behind at the lodge, never returned. A search began the next day.

Lodge co-owner Dean Bruemmer, who helped with the search Saturday, said he last saw his friend at breakfast. He said True gave no indication of a specific route.

"That's been part of the big problem with this. He didn't really say where he was going from here. There are a lot of trailheads up the road. We don't know which one he took," said Bruemmer, whose lodge is situated about four miles from the Gila Cliff Dwellings National Monument.

Though daytime temperatures in southwest New Mexico have been mild of late, temperatures have dipped into the mid-20s on recent nights. True was last seen wearing only shorts and a T-shirt and carrying a water bottle.

Fourteen search teams that were scouring the area Friday were supplemented with additional volunteer teams from across the state Saturday morning, state police spokesman Lt. Robert McDonald said. Teams were on horseback, using dogs and a helicopter and search plane were being used.

Still, as the days pass, the chances of a successful rescue diminish.

"We're going to do everything possible to cover as much ground as possible, but it's already been four days," McDonald said. "By no means are we going to give up, but time is of the essence as always in a search and rescue effort."

True, who has been friends with Bruemmer and his wife, Jane, for 10 years, would often visit their lodge while traveling between Mexico and his Boulder, Colo. home. As a result, Bruemmer said, True certainly knew the trail system well - which makes his disappearance all the more mystifying to everyone.

"I find it hard to believe that he's lost. I think that something happened, some kind of medical thing or an injury or who knows. Micah is a very strong, competent guy. I can't believe ... if he got turned around, by now he would have come out," Bruemmer said.

Michael Sandrock, a columnist who writes about running for The Daily Camera newspaper in Boulder, has known True for at least 20 years and has run with him. He called True a pioneer of the sport of ultrarunning, which involves running extreme distances, often on grueling terrain and many miles longer than a traditional 26-mile marathon.

True, he said, has a rebellious spirit but never sought to draw attention to himself even as he became legendary for his talents, which included "just going up and running for hours and hours at a time."

"He's just authentic and genuine ... Micah is a guy who follows his bliss," Sandrock said.

True is the race director of The Copper Canyon Ultra Marathon, a 50-plus mile extreme race that took place in Urique, Mexico on March 4. He has been featured in articles in running magazines and was a central character - known by his nickname, "Caballo Blanco" - in Christopher McDougall's nonfiction best-seller "Born to Run."

"He's such an integral part of the fabric of the ultra community," Sandrock said. "He's one of the stars .... the Caballo Blanco, he's a legend."