Jonny, 30, and Neil, 34, met in January 2008 on Waterloo Bridge: the former in such despair he was ready to jump, and the latter the unexpected good Samaritan who intervened.
With a promise that “things can get better” and an invitation for coffee, Neil successfully talked Jonny back to safety before they went their separate ways.
Eight years later, the two were reunited and have kept in contact as they prepared to run the marathon to raise awareness and money towards mental health.
They have met Prince Harry and the Duke and Duchess of Cambridge, who convinced the pair to run the race for Heads Together.
Jonny, who was diagnosed with schizoaffective disorder when he was 20, has spoken frankly about his own mental health struggles, which culminated in him standing on Waterloo Bridge contemplating suicide.
Neil, who spotted the lone figure of a young man standing still amid the commuters while walking to work, stopped to talk to him, telling him: “It’ll get better mate, you will get better.”
After a 25 minute conversation, the emergency services stepped in and the pair were swept away and back to their respective lives.
It was only in 2014, when Jonny launched a social media campaign to find and thank the stranger, that they were reunited.
He told his friend to run ahead of him in the race, because he recently suffered the loss of both his uncle and his grandmother. However, they ended up running it together.
Shortly before the marathon, he wrote: “Admittedly, I haven’t done the training I needed for it. It’s been a tough year so far with my relapse and going back into hospital, family illness, and most recently my Uncle and then Grandma passing away a few days ago. But I WILL complete the 26 miles! Even if I have to crawl some of the way!!
“This feels like such an important moment for mental health. They’re calling this the world’s first ever #mentalhealthmarathon It’s a privilege to be part of it.”