Ben Roberts weighs 250 pounds and would not be running in today's 5K race had he not learned of a friend battling cystic fibrosis. He has raised $500 for the cause.

"I hate running," said Roberts, 28, of Elkridge. "After 15 minutes, I feel like I can't catch my breath. But then I think, 'Come on, this little girl feels like this all her life. If she can get through it and not complain, then how can I not?'"

The 5K will be Sharon Williams' first run.

"I'll be out there hoofing it, in pain and agony," said Williams, 46, of Monkton. Her job as legislative advocacy manager for hereditary hemorrhagic telangiectasia, a rare blood disorder, has compelled Williams to wear an HHT T-shirt and become a jogging billboard.

"When I remember the phone call from a woman in Tennessee whose child had died in her arms from this disease, well ... it gives you a forum to run," she said.

Those who run for a cause are rarely saddled with a fear of athletic failure, said Joel Fish, director of the Center for Sport Psychology in Philadelphia.

"That extra layer of motivation shifts one's goals and takes the pressure off," Fish said. "People who run to 'give back' feel they succeed just by being in the race. There's really no way to lose."

Chris Brennan won't win the marathon, having pushed his twins in a double stroller for 26.2 miles. One son, Ryan, has autism; the other 6-year-old, Sean, has apraxia, a motor speech disorder.

Wheeling the pair through the streets of Baltimore will make his day, said Brennan, 37, an attorney from Princeton, N.J.

"Running marathons with my sons has strengthened our relationships," he said. "Last year, I did [Baltimore] with Ryan, whose autism makes meaningful family time difficult. But he laughs and smiles when we're running and takes uncharacteristic pleasure in watching the rustling trees or seeing a piece of paper being blown down the street."

In mid-race last year, a neighborhood kid Ryan's age jumped into the empty seat in the double stroller.

"Ryan laughed and gave him a hug," Brennan said, dumbfounded. "That spontaneity was a breakthrough."

Crossing the finish line today will be bittersweet, he said.

"For four hours, at least, Ryan will have been 'in the moment,'" Brennan said. "The race brings my son back into our world for a time."