What began as a way to shed extra pounds morphed quickly into a way of life for Tom Brown. The Glenelg resident began running 20 years ago, and it wasn't long before he got hooked on marathons.

So it is fitting that Sunday, as he prepares to retire from long-distance racing, Brown plans to run the same course for his last race as he did for his first one -- Washington's Birthday Marathon in Greenbelt sponsored by the DC Road Runners Club.

"When I realized it had been 20 years exactly since I had run this particular race and that this would be my 50th career marathon, it just seemed like this was the right time to end it," Brown, 57, said. "There was just the right chi."

While he has run in many prestigious marathons, including in Boston and New York, Brown's favorite run is the relatively obscure Bagel2Beer Marathon in Columbia.

"What I like about that race is there are not that many participants, maybe 45 or 50, so I can be on my own and get within myself," said Brown, who intends to "walk it fast" in September.

The starting point for the race is a bagel shop in the Wilde Lake Village Center and the finish line is near the rear deck of a Savage tavern, hence the name. There are no prizes for winners and no T-shirts for entering, just running for the sake of running.

"There's a party at the finish line, so it's the best of both worlds for me. I want other people around when I'm whining about my aches and pains," he said with a hearty laugh.

Brown said the physical challenge kept him returning all these years, and has influenced his decision to give up marathons.

"Running 26.2 miles is grueling. It is not the kind of event where you feel a burst of exhilaration as you cross the finish line," said Brown, a retired middle school physical education teacher who works in corporate fitness.

"Maybe 10 percent of the time it's pleasant -- you get in a groove and it's great," he said. "But usually there's not a whole lot of enjoyment. You feel crappy when you finish and it may take several days to get back to normal. The real reason to run a marathon is for the satisfaction of knowing you did something that wasn't easy to do."

But retiring from competing in marathons doesn't mean retiring from marathons altogether, said Brown, the fitness director of NBP Fitness in Annapolis Junction. He will continue to train marathon runners and is preparing three racers for a 50K run in Harford County in March.

Brown, who doesn't charge for his services, estimated he has trained 20 marathoners, including his younger daughter, Lauren. Brown's wife, Sandy, his older daughter, Kim Michelotti, and Lauren are preparing for a triathlon in April.

"I started to notice in the mid-1990s that running wasn't as rewarding as taking someone else to the finish line," Brown said.

His best time is 3 hours and 4 minutes, which he posted in 1991 at a marathon in Rhode Island.

"For me there's been a progression of thought since I started running marathons from, 'Can I finish it?' to 'How fast can I finish it?' to 'How many can I finish?'" he said. "When you reach the point where there will be no faster finishes, why not help others improve on their times?"

Brown, who taught for 31 years in Baltimore County before retiring in 2004, said he still has a teacher's instincts.

"Some people look forward to quitting and say, 'I'm so glad to be done with this,'" he said. "But I loved it right to the very end. I loved the kids and everything about being a teacher. Now, I have the opportunity to teach something else that I love."

Len Guralnick, past president of the Howard County Striders running club, has known Brown for eight years and called him "one of the funniest people I know."

"But the thing that is unique about Tom," he said, "is that he loves to run for the sake of running more than anyone I have ever met."

"The human body is set up to run," Brown said. "We have powerful muscles in our legs and butt from our days as hunter-scavengers, running to avoid being hunted by animals. The human body in motion is a beautiful thing."

When he trains racers, he runs alongside them most of the way and monitors their body language to know when they need to take a water break or when they should slow down, he said.

"The main aspect he helps runners with is pacing," said Michelle Garwood, who trained with him last year for her first long-distance race, the Marine Corps Marathon run through Washington and Arlington, Va., in October.

"I was a casual runner who always wanted to run that race. But without Tom's motivation, positive attitude and commitment, I wouldn't have had a clue how to do it," said Garwood, 39. "He is an inspiration to me."

Brown plans to keep active with shorter runs and will add hiking, brisk walking and biking to the mix. He has aligned himself with the Tour de Cure bike ride in Ellicott City, which benefits the American Diabetes Association. It is held at Meadowbrook Park in June.

"But if one of my grandchildren wants to run a marathon in another 15 years when they're old enough, I'll be there for them," Brown said. "Our species is always looking for a challenge and I'm no different from anyone else."