Martyn Allchurch, 41, who works at Forest Hill Pools in Lewisham, has been training alongside his friend, Kay Kapsalis, 60, who is battling pancreatic cancer.
Martyn, a fitness enthusiast from Bexley, has been helping Kay prepare for the marathon, which has always been on her “bucket list.”
Kay, a medical secretary from Dartford, is undergoing chemotherapy and has seen her energy levels fluctuate.
Martyn said: “Running the London Marathon has always been on Kay’s ‘bucket list.’
“When she got offered a place to take part, I started training with her to give her moral support.
“I wasn’t doing the marathon myself but then a slot came up for me so now I’m running, too.”
The pair will be raising money for the charity Backcare, which educates people about coping with and preventing back pain.
The organisation has been a massive support to Ms Kapsalis during her cancer treatment.
Martyn has been helping Kay to persevere with her training, doing shorter park runs or gym sessions when she’s been too tired for anything more.
He said: “I’ve also offered lots of psychological support and encouragement, which I am used to doing every day in my work as a swimming instructor.
“But it works both ways – Kay has also been brilliant at helping me to stay positive.”
When they take part in the marathon on April 27, Marytn said it is likely to be more of a walk than a run, but he will remain at Kay’s side, matching whatever speed she can manage.
Kay said: “I am very lucky to have been given a charity place to run for Backcare, especially as I have suffered back pain due to my cancer for which I am undergoing chemotherapy.
“I am extremely grateful that Martyn has agreed to train and run with me, despite the fact he is of course much faster than me.
“He has kindly agreed to ‘take one for the team’ and keep me company running the marathon and has also done so on my training runs even though I am rather slow.
“He puts up with my moaning for which he definitely deserves a medal.”
Martyn has previously taken part in the Three Peaks Challenge, where participants aim to run up Britain’s three tallest mountains – Snowdon in Wales, Ben Nevis in Scotland, and Scafell Pike in England – within 24 hours.
Having enjoyed the process of getting in shape for this year’s London event, he is considering entering another marathon elsewhere in the country later in the year.
He said: “I don’t want all the training to go to waste.”
Forest Hill Pools is run by charitable social enterprise GLL under the ‘Better’ brand.
Theresa Shangazhike, GLL’s general manager at the centre, said: “We couldn’t be prouder of Martyn taking part in the London Marathon to support his friend recovering from cancer.
“Martyn is always committed to helping charities and communities in any way he can.
“At work, he is always the first to step up when there’s a chance to help others, whether it’s through charity events or supporting local causes.
“His dedication to making a positive impact is truly inspiring and we’re fortunate to have him as part of our team.
“We’re all behind him as he takes on this challenge and I know he’ll give it his all, just as he does with everything else.”
