People who run the 26.2 miles of a marathon or the 13.1 miles of a half-marathon are presumably in good shape. They're not expected to encounter anything more than fatigue and the occasional pulled muscle during a race. They are certainly not expected to die. That's what makes the deaths of three men at Sunday's Detroit Marathon so shocking.

Jon Fenlon, Daniel Langdon and Rick Brown, ages 26, 36 and 65, all collapsed and died within 16 minutes of each other during the last two miles of the half-marathon. Brown fell and hit his head, but the cause of his fall and the deaths of the others are still unknown. All three were reportedly in good health, according to family members.

Since the weather conditions at the race were ideal by a runner's standard -- overcast and in the 40's, some speculate cardiac arrest is to blame. Veteran running coach and exercise scientist, Roy Benson, warns us not to react too quickly though and point the finger at running. "Running doesn't cause heart attacks. Heart disease causes heart attacks."

According to a University of Toronto study, the risk of cardiac deaths during a marathon is low. An estimated one runner per 126,000 will die from a heart attack. "Heart disease doesn't always show up until someone is straining and reaches their anaerobic area," Benson continues. "That can happen at the end of a race when runners are exhausted and trying to maintain their pace."

While many people will now wonder if running long distances is safe, it's important to remember that deaths at marathons are rare. In fact, you have a much greater chance of dying from choking on a piece of food than running a marathon.

"Running has so many benefits, physically and mentally," says Olympian, Jeff Galloway, who has trained over 250,000 runners. "I hear from people every day that running a marathon has changed their life in so many positive ways. Don't let the highly unusual occurrence in Detroit stop you from putting on your running shoes and reaping the rewards."

To make sure you train safely for a marathon, check out these training tips.