Milton Keynes-born James Tobia, 26, is a sports science student at the University of Bedfordshire, was a golf caddy for the Russian Olympic team, and runs marathons for fun.
He completed the Ironman course, a 2.4-mile swim followed by a 112-mile bike race and a marathon in 15 and a half hours.
James said: “It was one of the toughest competitions due to the weather conditions.
“I have never even swam in the ocean so it was a bit of a shock getting salty water in my face.
“I have done many marathons for charity but wanted to challenge myself to do the Ironman course.”
James’ trained for a year and a half, including while he worked in Rio de Janeiro during the Olympics.
He said: “Running around in the high heat of Rio must have helped with my fitness.
“I also spent last summer in Chicago with my fiancé, swimming in Lake Michigan.”
James has high hopes for the future.
He said: “I want to do the Tokyo and Boston marathons. Who knows, one day it might be Everest.”