BOSTON (AP) -- Temperatures rising into the 80s slowed the field at the 116th Boston Marathon and may have convinced as many as 4,300 runners to sit this one out.
Defending champion Geoffrey Mutai dropped out at the 18-mile mark due to cramping, one year after he won with the fastest marathon in world history. Mutai, who also won the New York Marathon in the fall, was hoping a repeat victory in Boston would earn him a place on the Kenyan Olympic team.
A total of 22,426 runners started Monday's race in Hopkinton in temperatures expected to be as high as 84 by the time the last of them finish their 26.2-mile trek to Boston's Back Bay. That's about 84 percent of the registered field of almost 27,000, with many of the no-shows expected to take race organizers up on their offer for a deferment into next year's race.
The largely unprecedented offer was issued in response to forecasts that called for temperatures rising from 69 at the start to 81 by the midpoint.
The heat didn't seem to be a problem for Canadian Joshua Cassidy, who won the men's wheelchair race in a time of 1 hour, 18 minutes, 25 seconds that is the fastest in history. American Shirley Reilly edged Japan's Wakako Tsuchida during a sprint to the finish in the women's wheelchair division.
But the runners were content to pace themselves.
The women went off to a slow pace, with a first mile of 6 minutes, 15 seconds and a three-mile pace that would bring them in at around 2 hours, 38 minutes -- the slowest Boston Marathon women's race since 1978. The men were more than a minute behind last year's course record at the 5K mark.
One year after cool temperatures and a significant tailwind -- perfect running weather -- helped Mutai finish in 2 hours, 3 minutes, 2 seconds for the fastest marathon ever, the heat had elite runners preparing for a slower pace and the recreational runners trying to figure out how to finish at all.
The wheelchair racers left Hopkinton at 9:17 under sunny skies and a temperature of 69 degrees, followed by the women's field at 9:32. It was 73 when the elite men and the rest of the field went off at 10 a.m.
The total field included 26,716 entrants, but 3,683 never picked up their bib numbers over the weekend. Another 607 who picked up their starting bibs did not show up at the start; they will be offered a chance to run in 2013 instead.
With forecasts of dangerous heat, the Boston Athletic Association warned runners to be alert for signs of heat stroke and dehydration and asked those who were inexperienced or ill to skip this race. The B.A.A. offered a limited deferment in 2010, when the Icelandic volcano eruption stalled air traffic in Europe and prevented about 300 runners from getting to Boston.
Five-gallon jugs of water -- twice as many as usual, organizers said -- were already lining the route early in the morning as volunteers and medical staff stood by preparing for the influx of hot and tired runners.
The Boston Marathon has had its share of hot weather, with the thermometer hitting 97 degrees during the 1909 race that came to be known as "The Inferno" and the 1976 "Run for the Hoses" that started in 100-degree heat and finished with spectators sprinkling winner Jack Fultz with garden hoses to cool him down.
The last on-course death of a Boston Marathon participant occurred in 2002, but the Chicago Marathon stopped its 2007 race after 3½ hours when a runner died after temperatures climbed to 88 degrees.
Hopkinton residents Ted and Nanda Barker-Hook have been handing out sports drinks, coffee, water, bananas, and sunscreen on the road leading to the starting gate for the past five years.
This year, no one was touching the coffee.
Recreational runners who did show up said they are prepared.
"You've got to know your own body," Mike Buenting, of Minneapolis, who has run 10 marathons, said as he waited for the starting gun. "You have to know how to hydrate and the rest will take care of itself."