"I've run some small marathons. [I] ran the Boston marathon. This is the biggest marathon I've ever been to. I wanted to see what a mega-marathon was like," said Rex Wier, who was running for charity.
"I'm running for World Vision," runner Jonathan Lin said. "As a personal goal, yeah, I'm doing it again. Last year, I didn't like my time. I did finish in five and one-half hours, but it was just really confusing and chaotic. So, I'm just hoping weather holds up, and everything goes okay this year."
Spectators were cheering in four official 'cheer zones' along the route. Organizers handed out megaphones and other materials to help support the runners.
The night before the race, runners carbed-up at a pasta dinner, and run officials were prepping for the city's warmer weather with water stations and medical support.
Marathon participants are running through 29 neighborhoods and cover 26.2 miles for the annual Chicago marathon.
Last year's marathon was marred by heat and humidity, which ended the event early with one runner dying. Many others were rushed from the course to the hospital.
Hydration is also important -- emphasized at the pre-race expo -- and underscored last year when 88-degree weather and a shortage of water led to collapses on the course. Three hundred and twelve people received medical treatment;184 of them were taken to the hospital. One of them died.
The 2007 marathon ended early with only 57-percent of participants finishing.
"We've added additional water stations, 20 now, medical support. An event alarm system to allow the medical director to communicate with participants," said Carey Pinkowski, Chicago Marathon director.
Many marathoners spent Saturday getting their bearings, doing some strange stretches or trying out high tech equipment.
"This is supposed to analyze my gait. I figured I'd check it out never done anything like thjis before," said Joe Fairchild.
Chicago's Olympic bid also figures in to this year's marathon. With runners from all 50 states and 110 countries, it's an opportunity to spread a positive, athletic image of the city.
"All the people cheering at you, that's going to be very exciting. Keeps you moving. After 24 miles it keeps you moving," said Carlos Garcia Medina, runner from Mexico.
"You see a lot of people from different places in the world, Japanese, Latin America," said Jocelyn Blandin, runner from Venezuela.
For Oriana Obiri... her father's memory is her marathon inspiration.
"My dad died of colon cancer 2 1/2 years ago since then a friend and I are doing races for the American Cancer Society and this is our first charity run for them," said Oriana Obiri, runner.
Last year marathoners raised more than $10 million for charity --and that's just those who registered their fundraising.
For most of the day Sunday, the marathon will essentially cut the city in half. Expect a lot of road closures. It's a little hard to get around.