Not the inclines of Heartbreak Hill that have doomed so many runners before him.
It was the downhill part of the race that makes his time of 2 hours, 3 minutes, 2 seconds ineligible for an official world record. In short: IAAF rules have deemed the oldest and most prestigious marathon in the world — long considered the one of the most difficult, too — to be too easy.
"You don't look at world records. You just go," Mutai said. "If you are strong, you push it. But if you put it in your head, you can't make it."
Mutai outsprinted Moses Mosop down Boylston Street to win by four seconds as the two Kenyans both beat Haile Gebrselassie's sanctioned world record of 2:03:59. Four men, including third-place finisher Gebregziabher Gebremariam of Ethiopia and American Ryan Hall, broke the course record of 2:05:52 set just last year by Robert Kiprono Cheruiyot.
"These guys obviously showed us what's possible for the marathon," said Hall, whose 2:04:58 is the fastest ever run by an American. "I was out there running, and I was thinking to myself, 'I can't believe this is happening right now. I'm running a 2:04 pace, and I can't even see the leaders.' It was unreal."
The IAAF must certify a world record, and it is unlikely to approve Mutai's feat. The international governing body's Rule 206 requires courses to start and finish near the same point in order to discourage downhill, wind-aided runs and the artificially fast times they can produce. (Boston has a net decline of 459 feet, though the course is dominated by hills going up and down.)
"We had a stunning performance and an immensely fast time here today," said Tom Grilk, the head of the Boston Athletic Association, after Mutai ran almost a full minute faster than the sanctioned world record. "We in Boston are well-pleased with what has happened, and that's good unto itself. The definitions of others, I will leave to them."
IAAF officials did not immediately respond to emails from The Associated Press seeking comment.
Although the organization's rules clearly disqualify the Boston course from a world record, it does list Cheruiyot's time in last year's race among the best times of 2010. Joan Benoit's 2:22:53 was considered a women's record in 1983, though that was before the IAAF refined its rules.
Mutai will receive a $50,000 bonus for the world best and another $25,000 for the course record to go with the $150,000 he and women's winner Caroline Kilel earned for the win. "This gentleman did both things, and we are honored to have played a part in his doing it," Grilk said.
Kilel won the women's race to complete the Kenyan sweep, outsprinting American Desiree Davila to win by two seconds in 2:22:36. Davila led as late as the final stretch on Boylston Street and ran the fastest time ever for a U.S. woman, five seconds faster than Benoit, who is now known as Joan Samuelson.
"We're knocking on the door," Hall said. "I mean 2:08 last year and 2:04 this year ... It's going to come; it's just a matter of time."
A year after Cheruiyot lowered the course record by more than a minute, almost 27,000 runners lined up in Hopkinton with temperatures in the high 40s and a 21 mph wind at their back — perfect marathoning weather. Kim Smith, a New Zealander who lives in Providence, took off at a record pace and led the women's race for more than 20 miles.
The men were more steady, and they were the ones to take down the old mark.
Mutai and Mosop ran side-by-side for the final miles before Mutai pulled ahead for good on Boylston Street. The 19th Kenyan winner in the past 21 years, Mutai raised his arms in the air and grinned.
"When I was coming to Boston, I was not trying to break the world record. But I see the gift from God," Mutai said. "I'm happy. I don't have more words to add."
Cheruiyot, who had been recovering from a car accident in Kenya, finished sixth. Defending women's champion Teyba Erkesso dropped out before reaching the halfway point.
The women's pack let Smith go, falling almost a minute behind. But 20 miles in, as she ran down Commonwealth Avenue in Newton toward Heartbreak Hill, she began to stutter-step.
Soon, she had stopped completely to rub her right calf. It was only for a few seconds, but when she resumed she had clearly slowed and the pack was upon her less than a mile later. Among them was Davila.
The American ran with Kenyans Kilel and Sharon Cherop through Chestnut Hill and briefly broke out of her rhythm to wave as the crowd began chanting, "U-S-A!" The three swapped leads down Beacon Street in Brookline, and Davila led even on the final stretch before Kilel outkicked her.
"It was the most excitement I've had in a race ever and just really carried me the last six miles," Davila said. "I felt that energy, and I felt comfortable at the front and pushing the pace because of that. It really just carried me through to the finish line."