With an estimated 1.2 million expected spectators cheering them on, the runners have trained, picked up their packets and numbers and are headed to the starting line.
"I'm just looking to get out there and put the training to work," said Josh Diver, who's running this year's Bank of America Chicago Marathon. "t's been a long training cycle and I just want to get out there and let it rip."
The Chicago Marathon is one of the world's largest races and will see participants from 135 countries. The 26.2 mile race winds through the city but starts and ends in Grant Park. Streets along the route will be closed off for the runner's safety.
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"I just love being here with all the people and watching everyone have fun with it," said Jessica Scannell. "Some are doing it competitively, some are just wanting to have fun. It brings the city together."
Carey Pinkowski, the race's director, said marathon running was considered an extreme sport 30 years ago.
"It's really become, over the 30 years, a social phenomenon," Pinkowski said. "If you look at the participation we have and the charity aspect of the event, that's really transformed everything."
Nearly one in four runners now race for charity.
The Chicago-based cancer support network Imerman Angels has over 240 runners on their team this year. The money raised represents a large portion of their yearly fundraising.
Judy Flavin is running for her son, diagnosed with a rare cancer when he was just 15.
"I started running. I'd never run more than 5 miles in my life, and it really pushes me," Flavin said. "On those hard miles, I think of my son. I think of all those people fighting cancer."