David Nerrow, 36, of Acton intends to run from Hopkinton to Boston today, just like 20,233 others in the Boston Marathon. He'll start a couple of hours later than everyone else, though, because at 10 a.m. he'll begin pounding out 26.2 miles on a treadmill at the Puma Store on Newbury Street.

Why?

He has declared a goal of raising $100,000 this year for research into cystic fibrosis, which afflicts his 4-month-old daughter, Emma, and the Puma stunt will bring in $1,300 for the cause.

But doing 52.4 miles today will also help him prepare for the June 28 running of the Western States 100, "the Boston Marathon of the ultramarathon community." Runners usually encounter snow near the beginning in Squaw Valley, Calif., and can expect to encounter 100 degree-plus temperatures by the end of the 100-mile trail run. Only half of the 400 entrants finish, Nerrow said.

Why do you run? "Stress release and fighting mid-age."

You describe the charity angle as a way to put your running addiction to good use. What's addictive about it? "I think a lot of endurance athletes get addicted to the extended endorphin high that comes only once or twice a year, unpredictably, deep into a long event."

The farthest you've ever run at one time? "Fifty-one miles."

The toughest part of doing that? "The physiological, mental, and chemical breakdown that comes after about 37 to 40 miles. In endurance racing, that's called when you bonk."

How many miles do you think you've run in total? "About 30,000."

Over how many years? "Twelve."

How many this year? "About 3,000."

What's the best part about running on a treadmill? "The ability to just zone out. You do not need to be conscious of your surroundings or the weather or anything."

What makes a successful distance runner? "You have to be a good sufferer. You don't have to be fast or intelligent. You just have to know how to suffer. And I'm good at that."