Darren Oliver from Stratford-upon-Avon was barely a mile into the race when he felt a severe pain just above his left ankle.
Thinking it was a bit of ligament damage, he soldiered on through the agony and made it to the end, raising £5,500 for the National Autistic Society in the process.
The swelling in his leg still hadn't gone down after a few days, however, so he decided to go to hospital where doctors were stunned to find that he had snapped his left tibia.
"I had a swollen ligament 10 days before and was told to ice it and rest it. Then on race day I set off and got a mile in when my ankle started getting painful, and as I got further round it became very painful," the dad of two told the Coventry Telegraph.
"When the doctor showed me the X-ray I couldn't believe it. They couldn't quite believe it either," he added.
"I had assumed that something was wrong with my ligaments or tendons, but I didn't think the bone was broken.
"The doctor thinks it was a stress fracture before I started and when I set off I made it worse and the bone just went."
Darren credits the incredible support he received from spectators for helping him pull off the superhuman feat of endurance.
"There were a few times where I wanted to give up but the crowds were amazing, shouting my name and pushing me on, and the adrenaline just kept me going," he said.
"My family and friends were all there cheering me on too so I just carried on."
Darren has not been put off running by the incident, and is already looking forward to beating his understandably rather slow time of six hours 29 minutes and 39 seconds.
"I'd love to do it again," he said. "If I can do the marathon in six-and-a-half-hours with a broken leg, imagine what I could do if I was fit and healthy with a good leg!"