Britain’s answer to Forrest Gump has fewer than 40 marathons to go to finish a 6,000-mile coast-to-coast run across the US dressed as a superhero.

Jamie McDonald, 32, has so far run more than nine million steps – averaging 53,000 a day – through 12 of the 20 states on his challenge, wearing a costume designed by a 10-year-old boy.

He is currently in South Carolina having completed more than 165 marathons and run over 4,500 miles.

During the run he is visiting sick children in hospital and has raised more than 120,000 dollars (£92,000) along the way.

The former tennis coach, from Gloucester, started his feat in April last year at Cape Alava in Washington state, the most western point of the US, and plans on finishing in Gloucester, Massachusetts, in March.

Running by himself and without a support crew, Mr McDonald will battle through desolate and lonely deserts, sub-zero temperatures and the constant threat of wild animals, injuries and the uncertain elements as he runs the equivalent of 230 marathons.

Since beginning his adventure, he has been caught in a terrifying flood in Junction, Texas, been mistaken for a Mexican drug runner, dodged striking snakes, scorpions and spiders, and been ordered to come out of his tent “with his hands first” by a police officer with his hand on his holster.

He has previously run across Canada dressed as superhero The Flash and this time he is running as his alter ego, Adventureman.

Jamie McDonald in Canada
Jamie McDonald in Canada (Jamie McDonald/PA)

Mr McDonald, who is in a relationship with fellow adventurer Anna McNuff, said: “It’s been lonely. I’ve doubted myself more than I ever have.

“I’ve slept in more bushes than I can remember, but also been fortunate to be supported often by the communities I’ve ran through.

“I can’t begin to describe the support I’ve had both on and offline, both in terms of donations and love.

“I’ve run through freezing rainforest downpours and the Arizonan desert summer.

“I’ve been caught up in a freak flood that devastated a town, camped in places surrounded by bears, cougars, mountain lions, spiders and snakes – if I didn’t have video proof, even I’m not sure I’d believe me.

“I’ve listened to some heartbreaking stories from the children I’ve met in hospitals over the last 4,500 miles.

“But each injury and each twist and turn has only made me stronger and more determined. I want to help, because I know I can. I know that we can.”

Mr McDonald, who suffered from a debilitating immune deficiency and the potentially fatal spinal condition syringomyelia as a child, spent the first nine years of his life in and out of children’s hospitals.

He added: “I’m starting the official countdown. Forty marathons to go – still the equivalent of running from John O’Groats to Land’s End in the UK and then 150 miles more.

“Forty marathons until I’ve completed the hardest challenge I’ve ever set myself. On which note, I have a little surprise to announce when I hit 20 marathons left.

“All I can ask is, please keep donating. Please keep sharing, and please keep believing that we can make the world a better place for sick kids everywhere.”

Mr McDonald came to prominence in 2012 when he cycled 14,000 miles from Bangkok to Gloucester on a £50 second-hand bike.

Weeks after returning to Gloucester, he set a new world record for cycling non-stop on a static bike.

In February 2013, two months after setting the new world record, he began his run across Canada.