Ryan Sutter Running the Boston Marathon for Ethan Zohn
Sutter will run Monday's marathon in support of Grassroot Soccer, the charity Zohn co-founded.
"Ryan is proof that you don't have to have a diagnosis to be affected by a life-threatening disease," Zohn, 38, tells PEOPLE. "I'm so impressed with his passion to help others. He stepped up for me before and is doing it again. ... To me, that's inspiring!"
Zohn ran the New York marathon in November, but a recent stem-cell transplant will prohibit him from participating in Boston's race.
According to Zohn, the procedure was "the most grueling test of strength and will that I have ever experienced."
"Every day I woke up afraid that I may not make it out alive," he said. "That being said, I have taken that fear and used it to keep me motivated to focus on the positives that have come from my experience."
More than 40 days after receiving the stem-cell transplant from his brother, Zohn said he is gaining strength and has recently started jogging and playing light soccer.
"It's not easy, but I'm getting stronger every day," Zohn tells PEOPLE. "I may not say it as much as I should, but all the love and support means the world to me."