Converging upon Chicago's McCormick Place, thousands of runners picked up their race bibs Saturday morning, eager to get off their feet and get some rest before the marathon.
The weather is predicted to be in the upper 70s and sunny. For now, the event's alert system is yellow, indicating some precautions need to be taken.
"Dress appropriately, make sure they have a hydration plan, bring the right clothing," said George Chiampas, the marathon's medical director. "The message is to those runners who are looking to set a PR that they need to adjust their expectations. Tomorrow may not be that day...you need to listen to your body."
Over 700,000 runners have participated in the Chicago marathon since its inception 40 years ago. It is one of the most popular races on the circuit.
"It's a celebration of humanity, the human spirit, people that have trained and committed themselves," said Carey Pinkowski, Chicago Marathon executive director.
SECURITY TOP PRIORITY IN WAKE OF VEGAS SHOOTING
Security is a major priority for organizers. Fencing is up, and the number of undercover officers has increased.
Homeland Security officials, the FBI and Chicago authorities put together a security plan for Chicago's marathon. ABC7 obtained a copy of the report, which detailed concerns about vehicles ramming crowds and attacks on public transportation.
"There is a great team here that understands events, and there is a great relationship with between the event producers and Chicago police and federal agencies so we share best practices," said Chicago Marathon race director Carey Pinkowski.
Organizers said runners and spectators should always tell authorities if they see something suspicious.
"There is no perfect security. Security comes in layers. It comes in improving the security of the people in the city of Chicago. And that is in fact what you're observing in the last few weeks," said Professor Robert Pape, director of projects on security and threats at the University of Chicago.
Larry Moon, 76, is celebrating a huge accomplishment. He has participated in every single Chicago Marathon. He has also run the Boston Marathon, and over years has noticed the security procedures change.
"You see all of the security out there before the race. Most of the runners are coming in from Michigan Avenue, you see Homeland Security, you see K9, everybody is out there," Moon said.
Moon said he always feels safe here and knows Chicago will be ready to handle another race weekend.
"Once I start running I don't even think about it," he said.