But Stewart, who will represent Team Scotland in track endurance events at the Commonwealth Games, is made of stern stuff. He might not look like a man of steel but he is the son of an iron man.
His father, Stan, competed in iron man events for Scotland and his mother, Caroline, is a keen hill runner. Their two sons are heavily involved in cycling with Mark targeting the Commonwealth Games and his brother, Kevin, working as the sprint coach. "He does not coach me but he is my mentor," says Mark of his sibling.
The upbringing in Dundee has shaped Stewart into a dedicated, even devoted, athlete. His early years involved competing in triathlons, with a particular fondness for swimming. He was put on a bike, too, at nursery and the wheels have gone round to such an extent that he has been rewarded with Commonwealth Games selection at such a tender age.
"It was always church and sport and discipline," he says, pointing out that his faith and sport played a large part in family life.
"I've always been drawn to cycling. I like going places and the freedom you get on a bike is nice."
He is also bolstered by a faith he talks about gently. "I go to church. Being a Christian relaxes me," he says. He adds of sport and belief: "We couldn't go every Sunday but those were my two staples. I go to church when I can but I don't have a problem competing on a Sunday."
His message is that his Christianity allows him to put his sport into perspective and to maintain an attitude that is as healthy as his svelte figure.
"I think it helps you through the bad days. At the end of the day I'm just riding a bike, not saving lives. It keeps you sane and allows you to train harder and get through the tough days," he says.
"I can switch off from it. It takes off the pressure. It's a peace of mind that keeps you relaxed and knowing all you have to do is give your best."
He has learned much from his parents, beyond the disciplines necessary to be successful in sport.
"I'd probably say my hero is my dad. I look at people in cycling and think they are people with two legs and two lungs and it's nothing I can't do. But my dad inspires me because he is a family man, and although he was very sporty it wasn't everything. His family was so important and although he was competing at a very high level he was very relaxed about it," he says.
The family who prayed together, trained and played together.
"My dad racing and mum racing was always family oriented. We would bring a picnic bag, there would be a great atmosphere at the races and it would feel almost like a holiday. We've all got a competitive nature but it was nice to mix it with normal life," he says.
His mother, too, brought a touch of science to the sporting endeavours. Her degrees included a thorough grounding in sports nutrition but Stewart believes he also learned from her approach to family life, praising her decision to step back from the sporting arena to look after her children.
He speaks highly of his team-mates in track endurance - James McCallum, Evan Oliphant and Alistair Rutherford - but is understandably delighted that after "a long winter" he has been rewarded with the glimpse of a golden summer.
"I had made the qualifications and done the times but it's nice to be announced in the team. It relaxes you, almost. I'm 18, it's a home Commonwealth Games and I want to make the most of it. The track events are the first few days so after that I want to soak it all up."
This should not be taken to mean that Stewart is merely at the Games for experience.
"I've no events coming up before the games so they are the next target. I've got two months to work hard and get even stronger. I'm from Dundee but I stay in Glasgow and train," he says, pointing out he has almost unrestricted access to the track at the velodrome. "I've gone from the youths to training with the best guys in Scotland. Two years ago they were just great riders who you looked up to."
He now has his sights on something tangible. "I'd love to medal," says the son of the iron man at the alluring prospect of gold, silver and bronze.