Paddy Lynas, from west Belfast, has beaten the odds to take part after ill-health forced him to leave his job and severely affected his daily life.
After receiving his diagnosis, he struggled with depression and was invited by charity Chest, Heart and Stroke to attend a local support group.
He was encouraged to manage his illness and start exercising.
Paddy told UTV the support groups changed his life - and that is why he is dedicating his marathon participation to the organisation.
"I would love to give something back to them. What they have given me, I could never repay them.
"So when I was asked would I fancy walking the marathon, I said I'll walk it, I'll run it, I'll skip it, I'll do anything just to get people to understand about Chest, Heart and Stroke."
Through the support groups, he pledged to get active and that he did.
"You make a promise that you're going to do something and then, you go back and tell them the following month.
"I went back and I said I walked a couple of miles, that was all right. The next meeting I was walking further and further and I was feeling better and better.
"After lots of months I was off depression tablets, anxiety tablets, the walking to me was my gift from that group."