The Buddies youngster kept himself ticking over during the summer by teaming up with older brothers Stephen and Paul for lung-bursting work-out sessions.
Stephen, 24 - also a former Saints player - was this week snapped up by new Sheffield United boss David Weir, while 22-year-old Paul quit the Paisley side at the end of the season after failing to break into Danny Lennon's team and is now on First Division Dumbarton's books.
And the youngest McGinn claims the three siblings' competitive edge has worked wonders for his fitness levels.
John, who returned to pre-season training with St Mirren on Thursday, told Press Association Sport: "Stephen, Paul and I have been going to the gym together and going for runs to keep ourselves fit during the summer
"The two of them are both naturally really fit but I need to work hard to keep on top of it. But having the two of them beside me made it a bit easier to push on a bit.
"The sibling rivalry came out and we all ended up pushing the pace up on the treadmill. It's good to have that competition to see who is the fittest."
And the 18-year-old Scotland youth international says he's been hitting the weights too in a bid to make sure he doesn't get pushed around in the new Scottish Professional Football League top flight.
"The gym work is massive because you are playing against such big players," he said. "There are guys in the league that are powerhouses. The midfield in Scotland is all about the rough and tumble - you have to earn the right to play.
"To compete with these guys you have to keep up on your upper body work in the gym."