Roger Comish will be running the 26.2-mile course in April for Leukaemia Care, a national blood cancer charity.
The 56-year-old IT manager is no stranger to marathons, having completed races across the UK and worldwide.
After a friend suffered an injury and had to give up their London Marathon place, Roger stepped in.
Roger said: “I’m running in place of a friend who fell in training, breaking their elbow and damaging their knee, which meant they were unable to complete the distance.
“They are still campaigning for Leukaemia Care and we have become a team of two.
“They will be there on the day too supporting me, so this is an extra special experience for me.
“I am so pleased to have this wonderful opportunity to run the greatest marathon in the world! There is no other marathon like London!”
Clare Brooks, who works with the Marathon runners at Leukaemia Care, said: “Our runners make all the difference to people who need that bit extra support.
“I’ve followed all our runners, including Roger, from the point of sign up and I can’t wait for Roger to cross that finishing line.”
Over the years, Roger has raised over £19,000 for charities, and hopes that the money raised at this year’s London Marathon will continue to do good.
“Leukaemia is a lonely personal island full of fears and strange things.
“Leukaemia Care can help patients through that horrible maze back to what life was like before this terrible disease struck.”
If you’d like to donate to Roger’s fundraising, head to his online giving page at 2018-london-marathon.everydayhero.com/uk/leukaemia-care.