Manzano, said Ed Coyle, director of the Human Performance Laboratory at the University of Texas, can consume 82.2 milliliters of oxygen per kilogram of body weight per minute  a capacity that, Coyle estimates, only 10 runners and 10 cyclists in the world can match. His large heart (revealed in an echocardiogram) means that Manzano can pump more blood and oxygen to his muscles than most men his size.

“They said I have the engine of a Ferrari in the body of a Pinto,” Manzano said, laughing.

Manzano’s hardscrabble roots might have contributed to his unusual toughness. He was born in the central Mexican town of Mojoneras, where education ceased by fourth grade, running water did not exist and electricity was practically unheard of  even in 1989 when his parents, Jesus and Maria Lourdes, moved 4-year-old Leo and his younger sister, Laura, to Texas’ Hill Country.

His father took a job crushing boulders at a quarry; Leonel was left to make sense of school.

In the seventh grade, a friend persuaded Manzano to try cross-country, but at home, sports were viewed as a waste of time.

But he had talent.

In the eighth grade, Manzano ran 800 meters in an astounding 1:55. As a freshman, he won the first of his nine Texas high school track and cross-country titles while training only 25 to 30 miles a week.

“My most important role with him was to make sure I had the entry fee paid and the bus gassed up,” Kyle Futrell, Manzano’s coach at Marble Falls High School, said, half-joking.

All the while, Manzano needed to help his family financially. He got his first job at 11. Later, his father would drop him off at school at 5 a.m. and Manzano would juggle practice at 6:15 a.m., his schoolwork and late shifts at an Italian restaurant until he became, in 2004, the first in his family to earn a high school diploma.

Then he became a student at the University of Texas, which confused everyone. “My parents never had any experience with college kids, and I didn’t know what I was supposed to do because I didn’t know anyone else who had been to college,” Manzano said. One day he packed his gear and said to his mother, “O.K., I’m out of here.”