They're pleased that cool weather is expected for the race.
That, combined with proper training, may bring record or near-record results.
The defending Olympic marathon gold medalist, 22-year-old Kenyan Sammy Winjiru, says he's excited to be in President Obama's home-town.
"It's my first-time to sit in America. And my first time to come to run in Chicago. And I'm very happy to see that guy Obama, where he's come from," Winjiru said.
The president enjoys competing in sports, but it's doubtful he'll challenge Winjiru to a race.
The marathon kicks off Sunday at 7:30 a.m.
Several streets will be closed for the marathon. For more information on the closures, visit www.chicagomarathon.com.