Kai the shar-pei became a global celebrity after he was left at Ayr train station with a suitcase full of his belongings in January 2015.
He was given a new home by Ian Russell, 53, from Newtonmearns, and a year on is helping his owner train for the gruelling marathon in April.
Ian said: “Raising funds for the Scottish SPCA is what it’s all about. I was lucky enough to rehome Kai and really want to give something back.
“I hand in food and bits and bobs for the animals but felt I should do something bigger and I wanted to test myself with a marathon.
“Kai loves coming out running with me when I train and it’s a piece of cake to him as he’s so young and fit.
“This will be my third marathon as I did one when I was in my twenties and one in my thirties, although this will be my first time running in London.
“Now that I’m in my fifties I think it will be more difficult but I’m not chasing a brilliant time, I just want to complete it one way or another and support the Scottish SPCA.
“I’m really looking forward to running through London and hopefully the great views will take my mind off the pain.”
Ian also spoke of life since rehoming Kai, saying: “It’s been a really quick year and we have bonded more and more as the months have rolled on.
“He comes to work with me every day and has a wee face he pulls if he thinks I’m going out without him, like when I’m putting on my aftershave.
“We still get recognised everywhere we go, even when we’re out hillwalking. Just recently we were in Glencoe when a lady stopped us.
“Kai is such a gentle dog and is great with everyone, including children. When we’re in the park he usually ends up getting chased by much smaller dogs though as he’s just so placid!”
Superintendent Sharon Comrie said: “We’re so excited to have Ian representing us at the London Marathon.
“It’s fantastic that he’s decided to take on this huge challenge for us and we wish him the best of luck in April.
“Anyone can see that Ian and Kai have a very strong bond and we’re delighted this special dog has found the true happiness he deserves.”
Anyone wishing to sponsor Ian can visit www.justgiving.com/Ian-Russell9/ or text KAIX99 followed by their desired amount £1/£3/£5/£10 to 70070