The 46-year-old has overcome more physical and mental challenges in the last 18 years than most face in a lifetime.
Driving to his Army camp at Weeton, Lancashire, in 1993 - just as he was about to go on leave - Billy was involved in a high-speed car accident. It put him in a coma and he had to have his right leg amputated.
The dad-of-two, originally from the Scottish Highlands, had just finished serving a two-year tour in Northern Ireland and had fought in the first Gulf War in Iraq.
But the horrors of war, countless injuries and the accident which nearly claimed his life have not been enough to put Billy off pursuing his goals, and the fitness fanatic is now preparing for a daunting challenge of endurance which would push any body and mind to the limit.
On July 8, Billy will plunge into the English channel in France and swim to Dover, to raise money for PoppyScotland.
But there will be little time to recover, as he plans to then get on his bicycle and head north, hopefully reaching Grangemouth within a week.
And in the final leg of his epic journey, Billy (pictured above) will run about 300 miles, using specially adapted equipment, from Grangemouth to his final destination of John O'Groats.
Billy, who works for Edinburgh's Lady Haig's Poppy Factory from his home, said: "Life is for living, you've got to get on with it. There's people in this world who come off with far worse injuries than I've had.
"We've got these brave men and women going anywhere in the world that the Government tells them to go and they fight. They get seriously injured, or worse. I just want to do something to help them."
Billy, who is married to Audrey and has twin children in Chloe and Ben, five, starts his gruelling training sessions at 4am up to six times a week, lasting into the evening.
To sponsor Billy, visit www.justgiving.com/Billy-Strathdee