March 23, 2011 -- Growing up in Idaho, Kelly Gneiting dreamed of running a marathon. But his weight, which reached 245 pounds in college, pushed him towards football and wrestling instead.

"I've always considered myself kind of an anomaly of an athlete as a big person," said Gneiting, who now weighs 400 pounds.

An athlete indeed, Gneiting is a three-time national champion sumo wrestler.

"Even though I'm big, I pride myself on being strong and tough," Gneiting said.

On Sunday, after only four months of training, Gneiting finished the Los Angeles marathon -- his second marathon in three years.

"When you do something once, people can think it's a fluke," Gneiting said. "But when you do it twice, hopefully you convince people that you're just that person."

Gneiting set out to inspire heavy people to break down the barriers that stand between them and their dreams. But in the process he appears to have also broken the Guinness World Record for heaviest marathoner, finishing the 26-mile course in a grueling nine hours, 48 minutes and 52 seconds.

"I told myself, 'Even if I have to crawl, I'll do whatever it takes,'" Gneiting said. "I wanted to prove I was tougher than the road."

After his first marathon in 2008, Gneiting pledged never to do it again. But on Sunday he shaved two hours off his time, despite heavy rain.

"The bottoms of my feet looked like white hamburger," he said. "There was a few times when a blister would burst and I'd feel it, and it just about caused me to collapse. And then I'd think, 'Oh my gosh, I still have six miles.'"

Gneiting, who works as a statistician at Fort Defiance Indian Hospital in Arizona, said he wishes he was smaller but refuses to let his weight hold him back.

"I certainly don't like being this big, but to me it's unacceptable to have low self-esteem," he told ABC News.

With Gained Weight Comes Loss of Self-Esteem

Roughly one-third of adults in the United States are obese, according to the Centers for Disease Control and Prevention.

"I don't know how or why I got bigger," Gneiting said. "I got married and focused on my studies, and just let myself go, I guess."

Despite his weight, Gneiting said his active lifestyle keeps him relatively healthy.

"My resting heart rate is 60 beats per minute. My cholesterol levels are fine, blood pressure is fine. I work out a lot. But that doesn't mean everyone should do it."

Dr. Daryl Rosenbaum, assistant professor of sports medicine at Wake Forest University in Winston-Salem, N.C., said carrying extra pounds on a long run can weigh heavily on the joints and the heart.

"A loose rule of thumb is that for every extra pound you carry over your ideal weight, it's an extra five pounds of force on the knees, ankles and feet," Rosenbaum said. "That increases the potential for injuries, like stress fractures, during a marathon and training."

But Gneiting's sumo training likely prepared his muscles and tendons for the extra force, and boosted his heart's ability to pump blood efficiently throughout his body.

"It's still remarkable that he's done this, but he does have the foundation of being an athlete," Rosenbaum said.

The risk of heat exhaustion is also higher when people are overweight.

"I think the key is you don't have to run a marathon to be active and get a great benefit. Do something fun that you enjoy," he said, adding that some overweight people like to work out in water to reduce some of the strain. "It doesn't have to be pounding or grueling. Start slow and set reasonable goals."

Gneiting said he wasn't sure he'd make it to the finish line Sunday.

"I thought at some point that my joints were going to give out, and there wasn't going to be anything I could do about it," Gneiting said.

He could barely lift his arm to wave to the hundreds of people cheering him on. But seeing one of his sumo friends at the 15-mile mark gave him the boost he needed to plod on.

Gneiting has four sons between the ages of 2 and 7 -- "future sumo champions," he called them -- and a 12-year-old daughter who is also overweight.

"I want her to look at her dad and say, 'I'm not going to let this weight inhibit me.'"

Gneiting: Don't Let Weight be a Barrier

Gneiting said he hopes his marathon effort will serve as a "penicillin shot" for overweight people with low self-esteem.

"I want to show them that just because you're overweight you can still be happy and pleased and proud of yourself and you can still accomplish something."

As for running another marathon, Gneiting said "No way."

"But I said 'no way' last time," he said. "Ask me in three years."