Mark Rowlands, 52, from Chailey near Uckfield, will begin his challenge on September 12.
He will hike 120 kilometres along the South Downs Way, starting from Exton in Hampshire and finishing in Alfriston.
Mark, who is married and has lived in the area since 2008, said the walk is to raise funds for The Royal Marsden Cancer Charity, which supports the world’s leading cancer centre The Royal Marsden in London.
The charity ensures that the centre’s nurses, doctors, and research teams can provide the best care and develop life-saving treatments used both in the UK and globally.
The 52-year-old told The Argus: “I have chosen The Royal Marsden Cancer Charity as my chosen charity as a friend of mine’s father used their services recently when he was diagnosed with cancer and I saw the great work they are doing.
“Cancer is a huge problem in the world and supporting a charity which is a world-leading facility that works on the prevention of cancer is such a good thing.
“I really want to promote this outstanding charity and I want to encourage others as well as to spread the word.
“I hope that this fundraising challenge makes a difference for those needing treatment and care as well as assisting with life-saving research.”
In the build up to the challenge, Mark, who has raised over £2,300 so far, has been taking on practice hikes with his two Welsh springer spaniels.
Last weekend, he trekked 20 kilometres across the South Downs with one of the dogs, Barney, in preparation.
Mark said: “I am feeling positive, nervous and very excited.
“I think if you don’t push yourself occasionally you become stagnant. Only problem will be my knees but I’m sure they will hold out for the duration.”