John Nicholas, 51, is running his fifth Chicago Marathon.
"I've been blessed to get a second chance," he said.
Twenty years ago, John broke his neck in a water skiing accident. He was paralyzed from the neck down.
"I have paralysis on the right side of the body from my right hand to my right leg to my right foot, and so, fatiguing is probably my biggest issue and overuse of the muscle," he said.
John has been running in support of the Illinois Spinal Cord Injury Association. Last year, he was given award for top fundraiser.
"I'm honored to run on behalf of them, and this is my 20th year coming back from quadriplegia. I mean, what a celebration," he said.
He hopes to beat his time.
"The first one in 2000 was 6:09 and then 5:30 and then 4:55 and then last year 4:32," he said. "This year, 4:20."
Curtis Harris is 36 years old and running his second marathon.
"Because it's exhilarating ," Curtis said.
Curtis is autistic and is running on behalf of Access Living. He hopes to beat last year's time which was 5 hours and 39 minutes.
"I'd like to finish at 5:30," said Curtis.
This is Halley Danisewic's first marathon.
"My background is actually in triathlon," said Halley. "I've always liked running short distance but never really had any intention of doing anything long."
Until she learned that an organization called One Step Camp had a marathon team.
"They run camp for kids with cancer, and I was lucky enough to go to their camps all throughout my childhood. When I found out about what they were doing, I decided that I should do it," said Halley.
"At first, my goal was to finish, but you know, now that I'm kinda going along. I would like to fall within the five to five and half hour range," Halley said. 15:05
If you want to show your support, cheer them on Sunday morning. The Bank of America Chicago Marathon starts at 7:25 a.m. with the wheelchair racers and at 7:30 a.m. the runners start at Grant Park.