Buying his first weapon -- a sawn-off shot gun -- at the age of 16, he was behind bars for the first of two jail terms just after his 18th birthday for his involvement in an armed robbery.
Also in that unit were the four men, who attempted bomb attacks on London's transport system in July 2005.
"It was probably at that point, I realized quite the degree of trouble I was in," he recalls.
But it was only after hearing a friend had lost his life, crashing his getaway car in an armed robbery while McAvoy was still behind bars, that he had his epiphany.
"If that hadn't happened, I'm sure I would have just gone back to it," he admits. "At the time, I was thinking about absconding and disappearing off to Spain.
"And I have absolutely no doubt in my mind that if I'd done that I'd either be dead somehow or else dead by default -- arrested for another armed robbery and with this time them throwing away the key. That would have been a death sentence."
His redemption -- also the title of his autobiography, which was published on 22 October
-- came in the form of sport and, in particular, Ironman, and he is using his dramatic U-turn in life to preach to others against a life of crime.
His past brushes with death, he insists, make an Ironman -- a 2.4-mile swim, a 112-mile bicycle and a marathon run -- that much easier to bear even though when he took up the discipline he could not swim and had not ridden a bike since the age of 12.
"I reckon 75% of it's mental," he says. "I've very nearly been shot dead by the police before, I've had 20 guns pointed at me at one time so when you're on the start line of a triathlon my thoughts are 'what's the worst that can happen?'
"I think I just have an ability to suffer more than other people. I can run into oblivion, as crazy as it sounds I can run until I'm unconscious, and I think the deep drive comes from the 10 years that I lost in prison."
McAvoy knew from a young age that he was destined for a life of crime, despite the best efforts of his mother.
"She wouldn't let me play with toy guns, I wasn't even allowed a water pistol," he says. "She was that concerned I'd get involved in that sort of life.
"She did everything she could but I witnessed the perfect storm. To me, it was like cowboys and Indians, and all these guys had money and it was from committing crime. But no one made me do it, it was my choice."
For McAvoy, the robberies were neither an adrenalin rush nor addictive, and he is adamant he does not miss that part of his life.
Instead, for him it was always a means to an end, that end being money.
"You do get the maniacs that love that aspect of it but I didn't get a buzz from scaring people, it was just very methodical and money based," he says. "I was always very ambitious as a child, and oddly it was about being the best."
He says he is driven by the shame of his past and that he is embarrassed both by his past actions but also by himself as a human being.
"I've now got a platform to do some good in my life," he says. As for the reaction to his book, he adds: "I've genuinely only had positive reactions so far. Sure, there are those that will always hate me for what I've done and that's understandable.
"But I've had a lot of messages from people who've even encountered worse than that, and say they've been inspired by seeing the power of sport. That's the message."
So dramatically has his life flipped that he is almost stunned by that past existence.
"I remember when I was taken away in the police car to prison the second time, I was thinking as I sat between two police officers 'I could kick the gear stick' and in my head I thought I might be able to flip the car and escape," he says.
"When they took me in I found out it was the high security unit and I was allowed in the communal area outside. The faces there were familiar and I realized it was Abu Hamza and the London bombers.
"It's odd as two weeks ago I'd been reading about them in a newspaper in Spain -- I was still suntanned at the time -- and now I was sharing this small space with them."
Hamza befriended him immediately with Weetabix, milk and a copy of the Koran, the last of which he rejected.
"It's the biggest book I'd ever seen but I was like 'honestly mate, I've got stuff to read in here.'
"But you'd quickly forget what he or the bombers were in for as they'd suddenly be talking about Thierry Henry or Arsenal or Chelsea, and you'd quickly realize that a lot of their background was the same as mine."
Exercise was his way out. It started in solitary in which he would do 1,000 press-ups, 1,000 sit-ups and 1,000 burpees, and then it extended to using the prison gym when he was moved from the high security unit.
A prison official saw his potential as he smashed record after record on the rowing machine, and suggested if this was what he could do on a prison diet, with limited training and electrolyte drinks self-made using Ribena, salt and sugar, he might have a sporting future.
In his first Ironman with just six weeks training, he finished the course in 11 hours but has slashed that time and, with a sponsor on board, genuinely believes he can take on and beat the world's best at the European Championships in Frankfurt next year.
"I'm a big believer that if you say something enough, it will happen," he concludes.
Given the manner in which he has turned his remarkable life around, you wouldn't bet against him.