Lewis Lathey ran 26 miles, about the marathon distance, from Brighton Pier to Horsham Park via Shoreham and the Downs Link path to Southwater.
The 15-year-old overcame a knee injury and temperatures well above 70°C, raising about £2,000 for Cancer Research UK as his nan, Carol Poulton, has cancer. Dozens of his friends and relatives cheered him as he finished the trek at the human nature garden in Horsham Park.
Lewis, who attends The Forest School, told The Resident: “Completing this run is something I will never forget. I’m so happy I’ve done it. It means the world to me.
“I got to the first stop after nine miles, had a two-minute break and was off for another four miles. But I really started to hurt during the third leg and everything was hurting on the fourth leg – my knee, quads, ankle, shin, everything. “I had to stop after about 21 miles and my dad put some deep heat cream on, but I was motivated all the way and thinking about being here (Horsham Park) at the end.
“My nan had cancer four years ago and was cleared two years ago, but it came back. She has been raising money for a cancer charity in Surrey, called Grace, for about three years and has raised about £20,000.
“My nan has been probably the most supportive person out of everybody. She has been such an influential person in my childhood and what she has done to raise money has been magnificent. When she got rid of the cancer the first time, I always said I would help her out if it ever came back. I’m just keeping my word.”
Mrs Poulton, 68, from Reigate, said: “I’m really proud of Lewis for what he’s done, very tearful and emotional because he says he’s done it for me. My cancer has returned, so this run has come at a very emotional time.”
Ten other people ran or cycled at least part of the route. Dad Garry cycled the whole way, while senior assistant head teacher Mark England and other teachers at Forest were also involved. Mr England, whose dog Teddy also ran the first nine miles, said: “I did this to support Lewis and because it’s a great cause.”
Lewis suffered a blow in May when an injury to the ligaments in his left knee, sustained while training in Horsham, prevented him from training for three weeks. Partly because of the setback, the event was pushed back from August 25 to Saturday, September 15. The three-week delay also enabled Lewis to maximise sponsorship at The Forest School, where he is a senior prefect.