It's a tough race for most runners, but it's unlikely that anyone will face a bigger challenge than E.J. Scott, a former, self-acknowledged couch potato turned marathoner. He is also legally blind.
"Centrally I can see OK," Scott told ABC7. "If I'm looking at you, I can see you OK, but then everything else starts to fade away."
EJ has choroideremia, a rare disease that degenerates the retina. It's hereditary. His grandfather went blind from it. Other members of his family have it.
"I have nephews that have this and I love them to death, actually, I have a picture," he said.
Little guys, who are unaware. They are, in part, why EJ runs. To raise money for the fight against a disease with no known cure.
EJ has run a marathon every month of this year. Chicago will be his 10th. By the end of 2012, he will have done 12 marathons in 12 states.
The sun is wicked on his eyes, so he runs with a blind fold on. And there is a guide running with him. They share a hand-hold, or the more recent preference is a towel.
He doesn't like to talk much when he runs, but he welcomes the guide's encouraging words.
"Twenty miles in, I start getting grumpy, I start thinking, this is horrible. They say you're doing great, you're great. OK, I guess," he said.
EJ finished his last marathon with his best time. Just shy of five hours. But he is ever mindful of a more important clock.
"Time is not a luxury that people in my situation have because every day we're losing more sight, and it's affecting more and more people, but if we can find a cure soon enough, it could be an essential cure for my nephews," EJ said.
While he had vision problems going back to his mid-teens, EJ - who is now 36 - wasn't diagnosed until his 20s. His knowledge of and fund-raising for the fight against choroideremia is all packaged on his website, www.ejscott.com.
So Sunday, it's Chicago, November in New York. December in Las Vegas will complete 12 marathons in 12 months in 12 states. And then, he'll stand down a bit.