Are you marathon material?

Jacqueline Stenson MSNBC contributor

Jacob Havenar ran his first marathon in 2000 with some soccer buddies who were looking for a new challenge. His motivation to keep up with the guys — and earn bragging rights — helped him make it to the finish line. But his main drive came from deeper within.

"It's nice to be able to say you've done a marathon," he says. "But for me, the part that meant the most was the sense of personal accomplishment. It changed my life. It made me feel like I could do anything in the world."

Like Havenar, more and more first-time marathoners are getting in the race. Statistics from USA Track and Field show that more than 400,000 runners now compete in an estimated 400 U.S. marathons each year, up from about 236,000 participants in 1990.

Marathon running is increasingly popular for several reasons, Havenar and others say. Some people are inspired by success stories common in the media and want to achieve such a big personal goal. Some are hoping to improve their health or lose weight. Others want to do a good deed; more participants are now competing to raise money for their favorite charity.

And why they decide to participate may make a big difference in whether they'll succeed, according to new research by Havenar, now a doctoral candidate in physical activity, nutrition and wellness at Arizona State University.

Going the distance isn't for everyone. Of 106 first-time marathon hopefuls embarking on a 20-week training program, 75 dropped out before the tenth week of training. Thirty-one completed training and crossed the finish line of the race. That's a 71 percent drop-out rate — steep for any fitness program.

When Havenar compared finishers to drop-outs, he found some patterns. "Those who dropped out were more motivated by a desire to lose weight and to gain social recognition," he says. Finishers, on the other hand, seemed to be driven more by desires to achieve a personal goal, boost their self-esteem or find meaning in life.

The findings aren't all that surprising given how much effort and sacrifice go into marathon running. Competitors often go to bed early and get up at the crack of dawn to run, run, run. They spend time away from their family and friends, and fight through injuries. All of the finishers in the new study, presented recently at a meeting of the American College of Sports Medicine in Denver, had experienced an injury at some point.

But aside from misguided motivation, there are other reasons why not everyone is cut out for marathon running, notes Dr. Malissa Wood, a cardiologist at Massachusetts General Hospital in Boston and four-time marathoner.

Wood says that because marathons are now more embraced by amateur runners — what she calls a "whole 'nother breed of cat" — participants are more likely to be older (the average age of participants is 38), less healthy and more unprepared for the stresses of the race. People have died of heart attacks during marathons, including two older law-enforcement officers who collapsed during this year's Los Angeles Marathon.

People over 40 need to be especially careful they don't have uncontrolled heart risk factors, including high blood pressure or cholesterol, she says. Smokers also are at increased risk as are those with diabetes or a family history of heart attack at a young age. People with any kind of heart abnormality should be sure to get a doctor's clearance.

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