The Ethiopian holds the 5,000m and 10,000m world records on the track.
Jeptoo will chase history as she looks to break 2:20 for the third time.
There's a lot of information to gather (and analyze) from 39,000 runners.
It took Maickel Melamed 16 hours and 46 minutes to complete the race.
The Kenyan smashes the course record and runs the fourth fastest marathon in history on Sunday.
Four of the runners in the men's field have run some of the fastest marathons in history.
Race organizers have stepped up precautions in light of the events at this year's Boston Marathon.
The 48-year-old wants to qualify for Boston.
A strong international field will line up in The Windy City on October 13.