Ben Kaye would have had a fat chance of completing a marathon when he weighed more than 32 stone. So finishing Sunday's race in four hours 37 minutes was particularly satisfying for the slimmed-down 34-year-old.
Mr Kaye - who grew up in Maida Vale but has relocated to Vancouver - had his stomach stapled in 2002. Within a year he had lost half his body weight. In 2009, he decided to get fit.
Canadian colleagues persuaded him to enter a 10km fun run and the experience sparked a passion for running.
"One day I decided to see what would happen if I continued running," he said. "I managed a half-marathon without any training. I was elated."
His London run earned £4,500 for the Nightingale home in Clapham, where both his grandmothers were cared for.
"It is a vibrant, warm and sunny place with beaming smiling faces all around," he said. "The amazing staff work tirelessly so that people can live the last days of their lives with dignity, while enjoying themselves and retaining their independence as much as possible.
"I'm still a big boy - 89kg [14 stone] - so there was a lot of me to get around the course."
Although he had enjoyed the experience, he does not plan to repeat it. "This was a 'bucket list' event."