It wasn't the speed, the physicality, or the fact that he was now the enemy of players four, five, even six years older.
What proved most challenging in Glenn Gawdin's initiation to the pro ranks was the blunt realization that reputation meant, well … squat.
"It hit me right square in the face," Gawdin said following the first day of practice at Flames Prospect Camp. "Like a freight train.
"Going into the year, I didn't really think about having to start all over again, but there I was.
"At the bottom of the totem pole in Stockton.
"It was like my first year in Swift (Current, of the WHL). Fourth line, having to fight for ice time and to the play in the situations you think you deserve. It took some time to let that sink in, but being a rookie, you've got to earn your ice, no matter what.
"I thought I knew more than I actually did going into the season - and that's normal for a young guy, I think - but once the reality of it all sank in, I did what I do best.
"I went to work."
Originally drafted in the fourth round, 116th overall, by the St. Louis Blues back in 2015, the B.C. native was left unsigned because there wasn't a spot available with the Chicago Wolves of the American Hockey League - a shared affiliate between the Blues and Vegas Golden Knights.
Gawdin re-entered the draft, but went unclaimed, putting his future on ice.
Or so it seemed.
On Nov. 16, 2017 - two-and-a-half months after impressing at main camp in the second of two summertime invites - Gawdin signed a three-year entry level contract with the Flames.
Six months later, after a stunning, 125-point overage campaign, Gawdin captained the WHL Broncos to a league title and an appearance at the Memorial Cup, in addition to earning the Western League's MVP honours.
"It's an incredible story," said Heat head coach Cail MacLean, who had a close-up view of the pivot's gritty work ethic last year. "There's a maturity about him that stands out. He's humble in his work ethic and that, alone, propelled him to have a great year with us."
In all, Gawdin put up 11 goals and 38 points in 64 AHL starts - the majority of which came in the back half of the campaign after gaining the "trust" of the coaching staff.
"He's very cerebral," MacLean said. "Players like Glenn, they need to understand that if they're in Stockton, if and when they eventually do go to the NHL, they're going to be expected to be really good when they don't have the puck.
"There's no one that will play that strictly offensive game, moving up the ladder like that. They'd have gone straight to the NHL, if so.
"That's what makes Glenn a good prospect. He understands that, and dedicates all of his time and attention on developing every aspect of his game.
"Are there things he needs to work on? Of course. Like everyone.
"But at this rate, he'll run with his opportunities."
Now at Flames camp for the second straight year - and with a pair of all-rookie Battle of Albertas on tap, beginning Saturday in Red Deer - Gawdin hopes to impress by showing off what he learned in Year 1.
Because, no matter what he accomplished late last year, that, too, means little until the bullets fly for real.
"I'm going out to prove myself," Gawdin said. "To my teammates, coaches - to everyone here.
"I know there's a place for my game at the NHL level. But it's up to me to get it there.
"Being a centre, I take a lot of pride in winning faceoffs, and every year, it seems like there's an even bigger emphasis on that area of the game.
"If I step up and show the coaches how reliable I can be in that area - and at the same time show how smart and responsible I can be on both sides of the puck, take advantage of my opportunity on special teams - I'll have accomplished my goal.
"It's a big year for me and I really want to take that next step."