CHICAGO (WBBM) — Among the 45,000 runners in the Bank of America Chicago Marathon on Sunday will be 10,000 people who are running for charity.

WBBM Newsradio’s Steve Miller reports on one competitor – and the 3-year-old son who’ll be waiting at the finish line.

“The first 24 hours was shock, basically,” Dan Plyter of Evanston said of the day he and his wife Susan found out their 15-month-old son Will had leukemia.

“Then there was about a seven-month time where it was just difficult. Emergency room, medicine, in and out of the hospital for different periods of time,” he said.

LISTEN: WBBM Newsradio’s Steve Miller reports

Next week, Will will be 3 years old and his mother, Susan, said he’s doing great.

“My sister sent a text message to us early on when we were in the hospital that said, ‘Tell Will to kick cancer’s butt,’” Susan said. “So his M.O. has been Operation KCB, which is ‘Kicking Cancer’s Butt.”

That motto will be on Susan’s shirt when she runs Sunday for the Leukemia and Lymphoma Society’s team.

“He sort of senses a few things and … he said to Dan one time, ‘Are you running for me?’” Susan said. “And we said, ‘Oh yeah, buddy, we are.'”

This is Susan’s second marathon – her first since Will’s diagnosis.

Dan Plyter calls it the “responsibility of the survivor.”

“There’s people who have been through harder things than the later miles of a marathon,” he said. “And people have written Will’s names on their arms and that had brought me to some points of emotional times.”