In his words, the former junior athlete had become a "pisspot party animal" at university and beyond. But then he turned his life around at 24, after watching his contemporaries race at the Melbourne Commonwealth Games in 2006 and realising what he was missing.
Five years later, St Lawrence is 20kg lighter and on the fast track to his first Olympics in London next year. He could earn his place in the team as early as tonight, when he defends the national 10,000m championship against the likes of Craig Mottram at the Zatopek meet in Melbourne.
But the transformation did not come easily or quickly, which puts St Lawrence in the ideal position to advise others who have fallen into a sedentary lifestyle and want to regain some of the fitness of their youth.
"I'd been pretty inactive for quite a long time but I'd studied exercise science and been an avid fan of running, so I kind of knew the level I needed to get to, but I was a long way from that," St Lawrence recalled this week.
He began his campaign by taking a hard look at his diet, concluding that he needed to put the right fuel into his body before he could expect it to perform for him.
"I did a diet overhaul - started trying to eat mostly fresh fruit and vegies, good carbs, smaller meals, trying to cut out takeaways. That was the initial thing," he said.
"There's only so much the body can do running-wise coming from being completely sedentary, so I started fixing up my diet, and was running just a couple of times a week to begin with.
"And I bought myself a mountain bike and started riding places rather than driving."
St Lawrence began from "absolute scratch" with just two half-hour runs a week, covering four to five kilometres (he will run 10km at more than twice that pace tonight).
"It took a couple of months for my body to adapt and start shedding the pounds and being able to put up with the extra running," he said.
"Early on, I jumped in a bit too quickly and tried to do some hill repetitions (in the Blue Mountains, west of Sydney, where he lived) and I got through two of them and was on the ground throwing up and I got a migraine afterwards that lasted for hours.
"My body really wasn't ready for that high-intensity exercise. But I got up to running every second day. I had a little rule with myself that if I hadn't run yesterday, then I had to run today. I found that was a good little mind-trick.
"In the days I wasn't running, I was riding. They say you shouldn't increase the kilometres by more than 10 per cent a week, so it was just a gradual build-up."
However, the bigger challenge for St Lawrence was resisting the temptations of the party crowd who had become his social circle in his hard-drinking days.
"Initially I found it really hard not to go out drinking and to get to bed at a reasonable hour, just because of the friends I had," he said.
"So I started telling my friends what I was trying to do, and tried to get them to understand it wasn't anything personal. It wasn't that I didn't want to have a beer with them, I just didn't want to have beer. I'd be out with my friends and almost shaking because I didn't know what to do with my hands.
"If I was going out, I set myself a four-beer limit, and I would try to be home by midnight. I'd have a couple of really good weeks, and then I'd have a blowout on a weekend. But slowly those blowouts happened less often, and my friends started to understand."
At the end of the 2006, after months working on his fitness, St Lawrence joined the Sydney training squad of coach Sean Williams - and that was a turning point.
"That's when running became part of my social life," he said.
"I would run with the squad four times a week, and sometimes we'd go out for dinner afterwards.
"In that squad, I was getting fitter, I was starting to drop the weight and I actually started to feel like a runner, rather than a pisshead party animal. It really helped to be around people who had similar goals."
It took him a full year of "false starts, and then a few good weeks, and then a blowout" to completely transform his lifestyle.
But four years after watching his former junior rivals compete for Australia in Melbourne, St Lawrence was at the Commonwealth Games himself, competing in the 5000m in Delhi.
This year he cracked the Australian 10,000m record in the US, setting a new benchmark of 27min 24.95sec, which puts him among the fastest 10 men in the world.
His 2006 self wouldn't recognise him.