Fitness fanatic Oliver Sanders is about to take on the biggest — and longest — challenge of his life. On Sunday, Mr Sanders, a 39-year-old father-of-three, will compete in the Forestman — a 4km swim, 180km cycle ride and 42km run. He hopes to complete it in less than 14 hours.

“It’s like jumping into the Thames at St Paul’s and swimming to Tower Bridge and back, then cycling to Birmingham and finishing it all off with a marathon,” Mr Sanders tells People. The Forestman, which takes place at the New Forest, Hampshire, is the final part of a three-part challenge that Mr Sanders set himself to raise money for charity. The first was the London marathon and the second a half-forestman.

“But this is the most challenging — and most stupid — thing I have ever done.” Is he worried? “Yes, I am worried, particularly about being on a bike for seven hours. I will just have to think about what I am doing it for to get me through my darkest hour.”

Mr Sanders is raising money for special-needs charity Kisharon and Putney’s Royal Hospital for Neuro-disability. He says: “Kisharon is very local to me and I see the good work it does. The Royal Hospital is closer to my heart. I had a childhood friend who got knocked over by a car, and he was cared for by the Royal Hospital before he died. So I want to do something in his memory.”

Mr Sanders has been training by running from his home in Hendon, North London, to Bank, where he works at AXA Insurance. “The only problem is, what will I do after the event? I’ll have to do something really dramatic.”

He is a member of the Ner Yisrael synagogue in Hendon.