HOUSTON -

The Houston Chevron Marathon is the largest single-day sporting event for the city. From runners to volunteers and spectators the crowd total is 250,000 people.  That takes a full year of preparations.

"The main objective is public safety from a health issue or possibly some type of terroristic threat; we oversee everything," said Assistant Chief Thomas Munoz with HFD Homeland Security. 

The Houston fire and police departments, along with other organizations, are working together at the start and finish line as well as throughout the route and staged in areas in case of medical emergencies, which was needed last year.

"We did have a runner that collapsed. Within approximately 40 seconds, we were doing CPR. We're glad to say that runner is living a full life," said Munoz.

Nearly 24,000 people will run this Sunday. There's a sizable group of elite athletes and everyday people who have their own story for running, including Mike Merino, a first-time runner in the marathon who is blind.

"I'm doing this for my kids to show them whatever the obstacle, a little work and courage, as well as faith … anything is possible," said Merino.

The winner of the marathon gets $40,000 and an extra $15,000 for setting a course record.

There is $50,000 on the line if a world record is set in the half marathon. Organizers say there is a good chance that will happen.