Steve Fanning, 35, will join a stampede of runners on the gruelling 26.2-mile challenge across the capital to raise funds for Fight for Sight, after his 93-year-old nan lost her vision.
He hopes to complete the course in just over four hours and raise £2,000 for the charity.
Mr Fanning, who was inspired after completing a half marathon, said: “I now would like to increase my challenge to try and raise even more money for Fight for Sight.
"I’ve seen the devastating impact that sight loss can have on someone’s life, as my 93-year-old nan has age-related macular degeneration which has caused her to lose her sight.”
With just over one month to go until the big day, on April 26, he will continue his unrelenting routine of 5am starts each morning to train.
The charity he is running for carries out vital research to prevent sight loss and treat eye disease.
Catherine Woodhead, director of fundraising at Fight for Sight, said: "We would like to wish Steve the best of luck with his training and for choosing to raise awareness and vital funds for Fight for Sight.
"A huge thank you to Steve and all our other runners for taking part in this year's marathon and literally going that extra mile for Fight for Sight.
"Good luck and we will be there to cheer you on every step of the way."
Last year, when the footballer ran the Royal Parks Half Marathon he completed the course in just under two hours and raised over £600.
To support Steve, donate online.