Moffat Academy head boy Jonah Lemon, joined by his dad Jim, will complete the run next month in memory of his cousin Nick, who took his own life in 2010.
The father and son duo are raising money for Papyrus, a charity dedicated to preventing suicide in young adults.
Jonah said: “Five years ago my cousin Nick took his own life, he was just 18. “Time moves on but we all still miss him greatly.
“I often think of all the things he could, and should have done, and how much of life he has missed out on.
“Nick had a passion for sport, so I am doing this is memory of him and in celebration of his life.”
He added: “I am raising money for Papyrus because they work to prevent young suicide.
“Nick had so much to live for and was so loved, the people he left behind will always feel helpless and confused at why he did what he did, but raising money for a charity that could help to prevent the loss of another young life feels like an important and positive thing to do.”
The marathon is a first for Jonah, who turns 18 next week and will be going to St Andrew’s University later this year.
He said: “Approaching the age that Nick was when he died has made me think about how fortunate I am and the possibilities that exist for an 18-year-old embarking upon the next chapter of their life.
“I want to make the most of every minute, for me and for Nick.”
He added: “I will never understand the decision that he took but I am determined to make the most of every opportunity I have.
“Running the marathon will be a great achievement. Raising money in memory of Nick, to help others, makes it all the more important.”
And raising awareness and funds for Papyrus is doubly important for dad Jim, 42, who is a consultant clinical psychologist at DGRI.
The London and Edinburgh Marathon veteran is encouraging his son as they set out to raise as much money as possible.
So far nearly £400 has been raised. To donate visit: www.justgiving.com/Jonah-Lemon