Are you marathon material?

Beginners also need a carefully structured training program. In her research, Wood found that amateur marathon runners who trained fewer than 40 miles a week showed more signs of heart trouble after the 2003 Boston Marathon than those who had trained more. The hearts of those who trained fewer than 40 miles a week didn't relax as effectively at the end of the race and the pumping function was decreased. The long-term implications are unclear.

Wood recommends that amateurs consult a doctor immediately if they notice any symptoms. "Listen to your body," she says.

Cedric Bryant, chief exercise physiologist at the American Council on Exercise, says anyone considering competing in a marathon or triathlon should set smaller goals first.

"It's probably a bit ill advised to go from couch potato to marathon runner in one fell swoop," he says.

Instead, start with a 3K, then do a 5K and then a 10K. Work your way up to a half marathon and eventually a full marathon, he says.

That will not only build your physical skills but also your mental ones.

"There's a lot of mental toughness and having to go through adversity to get through," says Havenar, who also coaches marathons and triathlons. "By mile 20, your legs will feel dead and you're gonna hurt. The last 6 miles are mental."

He says that while he's thrilled that marathons and other endurance events can inspire people to fitness, he worries that some people will aim too high initially and go too fast, setting themselves up for failure. Once they fail, they may go right back to the couch.

Staying in the race Check out these links for more information on marathon safety: Hydration recommendations from USA Track and Field American College of Sports Medicine (ACSM) guidelines on fluid replacement ACSM heat safety advice

"It could be a big hit and make it harder to get back on the proverbial horse," he says.

But the rewards can be great for people who stick with it, Havenar emphasizes.

Fueled by his own marathon success, Havenar has raised the bar even further. He now regularly competes in triathlons and last year he did his first Ironman, a grueling event that involves a 2.4-mile swim, a 112-mile bike ride — and then a full marathon.

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