Forty-five thousand runners from around the world are registered. It's one of the largest races in the world.
"I've done New York. So I thought Chicago would be next in line," said Pablo Sordo, runner.
The eyes of the sports world have been on the Chicago area for two weekends in a row, with Medinah hosting the Ryder Cup last weekend and Notre Dame and Miami playing at Soldier Field Saturday before the marathon Sunday morning.
"This is gonna be back-to-back sports with a huge impact for the city in economics," said Mayor Rahm Emanuel.
Many of the runners are also football fans.
"I'll watch it in a sports bar and I'll root for Notre Dame?I'll eat some pasta and watch them have some beers," said Suzanne Defruseio, runner.
Aside from economic impact for the city, the marathon raises millions for various charities. Open Heart Magic, which sends magicians to visit kids in hospitals, is one.
"They're definitely on my mind when I'm on the course," said Alyson Stevens who will be running for charity.
The race starts in Grant Park and will wind through the North Side to Addison before heading back downtown; then west to Damen before going back south to 35th; and finally to the finish back on Columbus Drive. More than a million fans annually line the route.
"The support, it is just tremendous. It is really gratifying, it makes you feel fantastic," said Brian Murphy, charity runner.