(CNN)Steven Quayle had little time to train for the London Marathon, but he still felt pretty good during the early part of Sunday's race.
Then, around 8 miles in, he stepped on a loose water bottle, injuring his right foot, calf and hip.
"I expected there to be pain along the way and carried on, thinking I would run it off," said Quayle, who's from Douglas on the Isle of Man. "The pain got continually worse however, and by mile 16 I was really struggling and stopped at a physio tent for assistance."
Most people probably would have given up. But Quayle, 29, said he was running to raise funds for Hospice Isle of Man and did not want to disappoint friends and family members who were supporting him through the run.
"They managed to do enough to get me going again. By the end of the marathon I'd had to stop at 4 or 5 physio points," he told CNN. "From the 16-mile point I was going an awful lot slower and got dragged around by the amazing crowd screaming at everyone."
Quayle also is an ambassador for Positive Bones, a charity that seeks to buy prosthetics for amputees.
"Josie Bridges is the founder of Positive Bones and she's lost a leg. She's a huge inspiration for me," he said. "At 22 miles a large part of me wanted to stop but I just thought, I feel like I'm on one leg here but Josie is actually one leg and she'd just power on; so I tried to power on."
He somehow managed to finish the run in under four hours, only to discover later that he'd broken a bone in his foot.
As he crossed the finish line, Quayle was hugely relieved.
"I was simply thinking, 'Don't cry in front of a TV camera -- and where's the nearest treatment tent?'" he said. "I was also incredibly surprised that I'd managed to complete the marathon in 3:57:33."
It was his first marathon, and also most likely his last, he said.
Quayle took a flight home, and as soon as he landed his sister took one look at his foot at the airport and told him they needed to go to the hospital straight away.
"I couldn't lift either of my feet off the floor when attempting to walk," he said.
At the hospital "the doctor told me I'd sprained my hip, torn part of my calf muscle and there was a line visible though a bone in my foot," Quayle said.
"Ultimately the way I looked at it was I'm here representing and raising funds for some wonderful causes who help people dealing with much tougher situations than myself for much longer than four hours," he added, "so the least I can do is put every last ounce of effort into it for them."