Tom Tronsdal grabs his house keys before asking the question. He knows the answer.

"Hey Mac," Tom asks, "do you want to go to the Magic Bridge?"

Three-year-old Mac lights up.

"Yeah!," he says. "Let's go!"

Within minutes, the Tronsdal family — Tom, wife Amanda and 3-year-old Mac — arrive at the Rillito River near their Midtown home. They reach a concrete bridge that spans the dry riverbed, and plan a race.

Tom gives Mac a head start, then chases him from one side to the other.

Tom's regular sprints with Mac hardly count as marathon training.

But they're everything.

Tom will run today's Boston Marathon, raising money as a member of "Tedy's Team," the foundation started by former Arizona football player Tedy Bru-schi — a stroke survivor — to benefit the American Stroke Association.

Tronsdal will cover the 26.2 miles in the world's oldest annual marathon for Mac. The Tronsdals' only child was born with hydrocephalus, an abnormal accumulation of fluid in the brain which can cause developmental delays or, in some cases, loss of brain function. Hydrocephalus affects 1 to 2 out of every 1,000 babies, and is the most common reason for brain surgery in children. The ASA funds hydrocephalus research.

"It's nice to have somebody from Tucson on my team," Bruschi said. "These people, like Tom, are my inspiration."

Early detection and emergency surgery has helped Mac avoid most, if not all, of the condition's worst side effects.

So far, Mac has only one physical limitation: The shunt installed in his skull means he can't play tackle football or go out for the wrestling team later in life.

"But that's not a big deal," Tom said. "Maybe he'll be a great cross-country runner."

"Lonesome Dove"

Tom's road to the Boston Marathon began with a treadmill and a vow to lose weight. Tom originally timed his training around college basketball games.

"I would run for the first half. Then I figured I'd run through halftime. After a while, I said, 'Why not? I'll just run through the whole game,' " he said.

Tom ran the Chicago Marathon, his first, before Mac was born. Following Mac's diagnosis, Tom realized he could raise money for a cause he believed in while keeping fit.

Tom, 38 and a lawyer, runs with Mac as often as he can. But his serious training takes place in the mornings, often as the rest of his family sleeps. He's out the front door at about 5:15 a.m, running a 5-mile loop near his home before returning home for breakfast with Amanda, 32, and Mac.

Tom runs with his iPod, listening to books on tape. "I just finished 'Lonesome Dove.' It was 36 hours, but it was really terrific."

Tom knocks out entire chapters on Sundays during his long runs. He starts at 4:30 a.m., running south on Mountain Avenue, through the University of Arizona and east to Reid Park before heading home.

Although he's run four marathons, today will be his first Boston Marathon, the most prestigious marathon in the United States and arguably the toughest. He is one of 44 people on Tedy's Team to receive a sponsor's exemption in today's race. Most of the field of 25,000 runners had to qualify by time for the elite marathon.

Tom hopes to finish in about 4 1/2 hours, 15 minutes better than his personal best of 4:45.

"I'm deceptively slow," he said.

A special kind of kick

But Tom's plodding has raised $15,000 for brain research. He hopes to raise another $5,000 today through donations to the American Stroke Association.

The inspiration for the training is Mac, who will track the event in Tucson with his grandparents.

Mac wears a lot of baseball caps to hide his shunt, the only visible sign of his hydrocephalus. The tiny tube sits under the scalp just beyond Mac's hairline and is hardly visible to the naked eye. The shunt connects to a series of tubes that help drain the fluid from his brain into his stomach, where it exits naturally through excretion.

The Tronsdals first learned of Mac's hydrocephalus during a sonogram five months into Amanda's pregnancy. Doctors explained that cerebrospinal fluid was building up inside Mac's brain in utero, and — for reasons they couldn't explain — was not flushing properly from his body. Hydrocephalus in newborns is often a symptom of a more serious problem, notably spina bifida or Down syndrome. Doctors were unsure what developmental delays awaited Mac.

Two days before the diagnosis, Amanda said she felt the baby kick for the first time. Mac kicked again, right on time, before each subsequent doctor's appointment. He was born five weeks early via Caesarean section on July 14, 2004.

He weighed 6 pounds 7 ounces, "and there was a sparkle in his eyes," Amanda said. "We just knew he was in there."

One day later, doctors performed life-saving surgery on Mac's brain. They implanted a shunt into the baby's skull to relieve some of the pressure on his brain. Mac began taking physical therapy classes immediately; he tested out of them last summer.

"He's fine to live a normal life," said Dr. Mary Cochran, Mac's pediatrician. "There's no reason to think he can't do whatever he wants to do."

Mac has settled into a life mostly free of doctor's appointments and physical therapy meetings. He attends Tucson Community School two days a week.

"Mac knows he has a shunt. He can feel the scars in the bathtub," Tom said. "In that way, he knows he's different."

The Tronsdals have a fan in Bruschi. The New England Patriots linebacker founded Tedy's Team three years ago to help raise awareness for stroke prevention. He will address the Tedy's Team runners before today's race.

Tom can see a bit of Bruschi in his son.

"Tedy's a real underdog, and I think Mac is, too," Tom said. "Mac is just a plugger. He's going to be really successful in whatever he does. He may never be the fastest kid, he may never be the strongest kid, but he's an awesome little guy. He truly is a miracle baby."

How you can help

• You can support Tom Tronsdal in the Boston Marathon by visiting tedysteam2008.