That's when leg cramps forced him to slow down and relinquish the lead.
"It's hot out there, in case you didn't know," he told reporters after enduring temperatures in the mid-80s to win the 116th Boston Marathon on Monday. "I knew it was going to be hot, and one important thing that I had to take care of today ... was really hydrate as much as possible. I guess my biology degree kicked in a little bit."
Singing religious songs as he trudged along the scorching pavement, the native Kenyan - a permanent resident of the United States - retook the lead from Levy Matebo in the final mile to cross the finish line in 84.8-degree temperatures with a time of 2 hours, 12 minutes, 40 seconds.
It was almost 10 minutes behind the world best established here a year ago by Geoffrey Mutai and the second-slowest Boston victory since 1985. Mutai, who was hoping a repeat victory would earn him a spot on the Kenyan Olympic team, dropped out after 18 miles with stomach cramps.
Instead, it was Korir who may have won a ticket to the London Games.
"To me, I think running the Boston Marathon is an Olympic event," he said. "I don't care what comes up after this, but I'm really, really happy to win Boston."
Sharon Cherop won the women's race to complete the Kenyan sweep, outkicking Jemima Jelagat Sumgong to win by 2 seconds in 2:31:50. The women's winner was decided by a sprint down Boylston Street for the fifth consecutive race - all of them decided by 3 seconds or less.
Cherop, who was also hoping to be selected for the Kenyan Olympic team, was third at the world championships and third in Boston last year.
"This time around, I was really prepared," she said. "Last time the race went so fast and I didn't know I was about to finish. I didn't know the course well and I didn't know the finish line was coming."
Matebo finished 26 seconds behind Korir, and Bernard Kipyego was third as Kenyans swept the podium in both genders. Jason Hartmann, of Boulder, Colo., was in fourth place and the top American.
"The pace wasn't blasting, so it wasn't anything that was over my head," Hartmann said. "There were so many times that you wanted to throw in the towel, but you just fought on. I don't think that anyone coming to this race really could say they were prepared for this heat."
Korir, a two-time winner of the Los Angeles Marathon, was the 19th Kenyan man to win Boston in the last 22 years. But he is hardly typical of the African runners who have come to dominate the event since Greg Meyer became the last American winner in 1983.
After starting college at Murray State - the Racers, naturally - he transferred to Louisville and graduated from there with a biology degree. He is hoping to receive American citizenship within a few years.
The winners will receive $150,000 apiece. Korir and his wife, Canadian runner Tarah McKay, run a foundation in his hometown of Kitale and have been building a hospital in the memory of his brother Nicholas, who was killed by a black mamba snake at the age of 10.
The heat slowed the leaders and led to warnings that may have convinced as many as 4,300 no-shows to sit this one out. Race organizers offered those who picked up their registration packets but did not start the opportunity to save a place in next year's race.
The largely unprecedented offer was issued in response to forecasts of high temperatures that rose from 69 at the start to a high of 89 by mid-afternoon, when recreational runners were still streaming across the Back Bay finish at the end of their 26.2-mile trek.
Crowds at the Copley Square medical tent were bigger than in previous years, with the smell of sunscreen and the sound of ambulance sirens in the air. Boston Athletic Association officials said their medical staff was helping more people and the busy period at the finish line medical tent arrived later than usual.
Buses that sweep the course for stragglers in previous years had returned empty but this year are coming in full.
"It is a very busy day, but it was the day for which people planned," B.A.A. executive director Tom Grilk said. "The god of marathoning, she smiled on us."
A total of 22,426 runners started the race in Hopkinton - about 84 percent of the registered field of 26,716 entrants. A total of 3,683 never collected their bib numbers over the weekend. Another 427 who picked up their starting bibs did not show up at the start; they will be offered a chance to run in 2013 instead.
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 beat the previous world best by 2 seconds. American Shirley Reilly edged Japan's Wakako Tsuchida during a sprint to the finish in the women's wheelchair division.
But Korir said that after coming from sixth place at Mile 20 to take the lead, he struggled to maintain his pace and Matebo went back in front.
"I started to get really bad cramps in my legs and needed to slow down and needed to slow down a little bit and ... I was then passed," Korir said. "Soon, I started to feel better and was able to pick up my speed again."
One year after cool temperatures and a significant tailwind helped Mutai finish in 2:03:02 for the fastest marathon ever, the heat had elite runners preparing for a slower pace and recreational runners trying to figure out how to finish at all.
Race officials 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. also 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.
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.
Those who did show up said they were 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."