David Ollerton, from Cardiff, has raised more than £6,000 for the cancer hospital which helped him.
Hill walking is strenuous at the best of times, let along when you're battling cancer.
But Velindre patient David Ollerton hasn't just tackled a couple of hills - he's bagged the set.
The 66-year-old, from Thornhill in Cardiff, has climbed all 33 of the 2,000ft hills between Brynamman and the English border - all while undergoing chemotherapy.
He hopes he can get himself a place in the Guinness Book of Records.
David is battling cancer for the fourth time.
His first diagnosis was in 2005 when he was told he had skin cancer. Two years later he was told he had Non-Hodgkin's lymphoma, followed by bowel cancer in 2010.
Then, last year, he was told his cancer has returned.
After two bouts of unsuccessful chemotherapy his latest spell began in June.
Since then he has been climbing the hills in South Wales. He has been joined on 23 of the 33 by friend Pete Hodge.
Each walk has included between one and three "tops".
He invited people to sponsor either one hill or the whole effort and has raised more than £5000.
His target was £2000 but, having completed the challenge with a climb up Pen Y Fan last Thursday, he has now raised more than £6000.
David Ollerton with his family
A keen walker, David had to time his walks during his chemotherapy.
It wasn't until the third week of each course of chemotherapy he had enough energy to walk.
"In a sense, I was better prepared having done them all before, because I knew the hills well. I knew I wasn't going to get lost but I only had one gear and very little energy."
When David described his challenge to a nurse at Velindre, where he's having treatment, she told him most people on his course of chemotherapy would struggle to walk anywhere, let alone up hills.
He said the challenge was a way to say thank you to the staff at Velindre.
"Velindre is just brilliant in their attention to trying to keep you alive," he said.
During his cancer battles, David said he has seen his treatment change.
"When I had chemotherapy five years ago it made me seriously ill.
"But now with the very costly anti-sickness treatment that has meant I have been able to do this.
"I couldn't have done these walks if I had been as ill as last time," he said.
"When you're fundraising for research you're putting something back into Velindre but hopefully helping someone else too.
"I am a Christian and part of that is wanting to contribute and care for others like others care for you.
"I have been a pastor at churches all my life and I am now on the end of pastoral care and people have been very good to me," he said.
Donations have come from around the world and the Wales-wide group which David is part of.
To add to the total visit: ww.justgiving.com/fundraising/David-OllertonBACK