Mike Francis, of Taunton based 40 Commando, took on the challenge after completing eight weeks of training.

Justine Jianikos, from Pontypool, Wales, has stage four breast cancer that has spread to her spine and lungs.

Mr Francis, 37, said of the challenge: “We were in pain, but it’s nothing compared to the pain Jussy is in”.

The royal marine, who was joined by two friends, said he had never done a marathon before and had not gone further than about six miles (nine km) in a “very long time”.

“My first marathon was on Wednesday,” he said.

“I just do three to four runs a week to keep myself ticking over, so I had to train for it and dig deep as anyone would who runs marathons.”