David Rowlinson will travel the 26.2 miles around London, while being pushed by his daughter and her partner.
David is taking on the challenge to raise money for Myositis UK - a charity supporting those with the condition.
He has Inclusion Body Myositis which affects around 6,000 people in the UK, causing chronic inflammation and wastage of the muscles.
It is an autoimmune disease for which there is no known cause or cure.
David has sourced a running wheelchair designed for speed and allows either one or two people to push.
David's daughter Jenny and her partner Jonny are currently training to run the distance while pushing the chair.
He explains how the idea came about:
"My daughter Jenny, she started running half marathons about half a year ago now and she had the idea of trying to get us an entry into London to raise some money for Myositis - which she managed to do.
So it was all her idea and I’m obviously very proud she’s doing it.
We plan to do the London marathon, and then the Guernsey half marathon in September.
After that, the idea is we will donate the specialty chair to the community so that anybody else who wants to use it for fundraising or a marathon will be welcome to do so."
They have almost doubled their fundraising goal of £3,000 on JustGiving.
Civil servants delay briefing politicians in Guernsey over hospital's escalating costs
Eco travel in Guernsey: swimming to work
Guernsey doctors will stop prescribing pain relief plasters
Candidates in Guernsey's 2025 election may not get a campaign grant