Organizers of the 116th Boston Marathon asked many runners to skip the race Monday because of unseasonably warm temperatures, the New York Times reported.
The marathon began Monday morning with temperatures in the low 70's, scheduled to rise to 80's before the runners made their way to the finish line.
Race organizers gave runners the option to defer their run until the 2013 marathon.
"For the overwhelming majority of those who have entered to participate in the 2012 Boston Marathon, you should adopt the attitude that this is not a race. It is an experience," race organizers said in an email to those registered to run.
The top women ran the first three miles at a pace that would have them finished in 2 hours and 38 minutes, the slowest Boston Marathon finish since 1978, the Los Angeles Times reported.
Although runners have to meet strict qualifications to participate in the Boston Marathon, race organizers urged non-elite runners to skip the race this year and try again next year.
"You've got to know your own body," Mike Buenting of Minneapolis told the Los Angeles Times. "You have to know how to hydrate and the rest will take care of itself."