The former Australia captain and World Cup winner will be pounding the streets on Sunday, April 21, with some of his work colleagues at Sky Sports, almost 12 months ago to the day since he was struck by a severe and life-threatening stroke.
Lynagh was rushed to hospital in Brisbane and stayed there for almost three weeks before being released.
The 49-year-old admits that he is lucky to still be alive and has almost made a full recovery - his peripheral vision is the only thing lost - in a relatively short space of time.
So now he will take on a massive challenge, when he and the rest of Team Sky For Stroke hit the streets of London, trying to raise money and awareness for the Stroke Association.
"I see myself as one of the lucky ones being able to go back to work - not all stroke survivors get that chance. That's why I'm supporting the Stroke Association," he said.
"I'm excited to try and help change that."
Lynagh and a team of seven are looking to raise at least £80,000.
The Aussie launched his martahon challenge last month along with James Gemmell, who came up with the idea of Team Sky For Stroke's taking on the marathon for charity - not expecting the man himself to be leading the way.
The pair, who can usually be seen on Friday and Saturday morning's covering the Super 14, and their Sky Sports colleagues are involved with a back-to-work project launghed by the Stroke Association.
"I had no idea that a stroke can happen to anyone at any age with about a quarter of all strokes each year happening to people of working age," explained Lynagh.
"Our main aim is to employ full-time employment Support Coordinators who can help liaise with Stroke survivors of a working age," he said.
Lynagh is also being supported by the club where he brought his glittering career to a close, Saracens.
He was at Twickenham for the Heineken Cup quarter-final win over Ulster, where a video explain what their former star is doing and where buckers were passed through the crowd for people to play their part alongside a legend.
You can support Team Sky For Stroke and donate £5 to the Stroke Association by simply texting STROKE 5 to 70300, or by visiting JustGiving.