Like anorexics, they say fitness obsessives can’t seem to stop. Celebrities do it all the time as they get in fighting shape for their next film ot TV show. And yet, mere mortals seem to be doing the same these days. More Americans are engaging in extreme exercise than ever before. According to Running USA, a nonprofit organization based in Colorado Springs, the number of marathon finishers has increased by nearly 50 percent since 2000. Yoga, meanwhile, has exploded — going from 4 to 20 million practitioners in the U.S. alone over the last 10 years — with a surge of interest in vigorous forms like Bikram (hot yoga) and Ashtanga (an athletic series). Add to that the proliferation of high-octane boot camps, spinning classes, and ballet workouts. Some aren’t content to simply get in 30 minutes or an hour but rather a couple hours a day working out. When is it too extreme? Read more.