There are 32 competitors -- 20 men and 12 women -- who are 18 years old, the youngest age the BAA allows to participate these days. More than half of them are from Massachusetts.

In last year's men's wheelchair race, Hiroyuki Yamamoto of Japan, a Boston rookie, grabbed the lead early, between the 3- and 4-mile marks, in Ashland, and slowly pulled away for the rest of the race. His time of 1:25:32 was a minute and 40 seconds ahead of the second-place finisher, nine-time Boston winner Ernst Van Dyk of South Africa. Both men are in today's field. Today's two wheelchair winners will pocket $15,000 of sponsor John Hancock's money.

Lest we forget, the men's wheelchair field also includes Josh Cassidy, 29, of Port Elgin, Ontario, who won Boston in 2012. His time that year, 1:18:25, is not only a course record, it's the fastest wheelchair marathon of all time. Cassidy, using a new a new chair, went out fast in last week's London Marathon, but hurt his back and eased up to save himself for Boston. He's been training and recovering at home in Canada since. He recently tweeted that he is still sore, but "should be good to go" today.

The wheelchair athletes are clear of the slowish controlled start, and are rolling down Route 135 towards Ashland.

Wakako Tuschida of Japan set the world record for women's wheelchair racers in Boston in 2011, crossing the finish line at 1:34:06. She returns this year after missing last year because of illness.

Next up at the starting line are the 22 handcycle racers, who are part of the BAA's mobility impaired program. Twenty of these participants are men; 2 are woman. Most of them will take well under two hours to travel the 26.2 miles to Copley Square.

Boston, with its steep uphills and downhills, is not considered a particularly fast marathon course. In Runner's World magazine, Ron McCracken called it "uniquely brutal." Only 18 of the 400 best marathons of all time have been run here, and eight of those occurred in the epic 2011 race, where perfect temperatures, a persistent tailwind, and an especially talented field combined to produced not only the fastest marathon of all time, the 2:03:03 by winner Geoffrey Mutai, but the second fastest ever, the 2:03:06 by Moses Mosop. Both men are Kenyans.

The American record of 2:04:58, by Ryan Hall, was also set in that race, and is the 38th fastest marathon ever.

The elite women are at the starting line in Hopkinton, jogging back and forth along a stretch of East Main Street (Route 135) to warm up.