The singer explained that he is running for Catch22, a charity which helps disadvantaged youngsters.
In a post on his Just Giving page, Young wrote: "I am very excited/nervous about running the marathon for Catch22.
"With each painful mile I stumble along in my training I think of the potential money and profile I am raising for a charity that is very close to my heart."
He then promised to wear a one-piece male swimsuit, made famous by Sacha Baron Cohen in 2006 film Borat, if he reaches his target amount.
Young added: "If I make £40,000 this year, I will do it in a mankini next year!"