Featuring David O’Brien and Samantha Gash. Unrated.

The urge to reach extremes has always been part of the overall obsession for marathon types. But thanks to carbon-guzzling technology and bizarrely rarefied new levels of competition, the athletes profiled here really go the extra mile, taking their running game to places where people probably shouldn’t even walk. Whether this is good for raising awareness of the planet’s vast yet fragile ecosystem or is just another expensive way to fetishize the body is left to the viewer to decide.

The 14 folks we spend time with are all on their way to the 2010 version of something called the 4 Deserts Grand Slam, in which contestants have a year to tackle epic wastelands of brutal hot and numbing cold in China, Chile, North Africa, and, finally, Antarctica. (One of the film’s producers, ultramarathon man Dean Karnazes, was the first person ever to complete the full desert ordeal, in 2008.) Of particular interest are Irishman David O’Brien, at 56 the oldest person to attempt it; 25-year-old law student Samantha Gash, from Australia; and ex-Brit-special-forces dude Tremaine Kent, 40, who recently lost his wife to cancer.

The initial trek across Chile’s Atacama desert is beautiful enough, in a lunar way, to suggest that things won’t be so tough. But subsequent treks across the Gobi and Sahara prove less forgiving—especially after one fellow passes out in the middle of nowhere, and one of the featured women gets assaulted by a local thug. The more mundane, but still serious, challenges involve failing footwear and other logistical issues.

A second documentary feature from director Jennifer Steinman, whose powerful 2009 Motherland followed six grieving American women to South Africa, the beautifully shot Desert Runners leans a little heavily on U.S.–style inspiration. But the new doc offers enough grit to stay real. And after a while you realize that the filmmakers are running the same courses as the runners, but (pace Ginger Rogers) backward and with cameras.