The urge to reach extremes is always part of the overall obsession for marathon types. But the half-dozen or so athletes profiled here really go the extra mile, taking their running game to places where people probably shouldn’t even walk: vast deserts of brutal hot and numbing cold in China, Chile, North Africa, and Antarctica. It leans a little heavily on U.S.-style inspiration but the doc offers enough grit to stay real.

Cinematheque, September 26 (4:45 p.m.); Playhouse, October 1 (6:30 p.m.) and 3 (4 p.m.)