For the Palmer Ridge High School star, it was just another day in his relentless pursuit of improving his craft.
"Just yesterday I worked out three times a day and they were hard lifts, doing everything I need to do," he said.
While he may have the look of a college athlete, Cullen is actually still trying to get college coaches to notice him. Among the teams he'd like to impress is the Colorado Buffaloes.
"CU is the school in Colorado," he said. "You've got (Colorado State) and Northern Colorado but when you think of Colorado and everyone thinks of Colorado, they automatically think of the Buffs most of the time. They're a school in a good conference. The whole program and the size of the guys and the coaches and the facilities they have there are really nice.
"I've definitely been in contact with CU a bit. I know their new D-line coach, Kwahn Drake, has been following me on Twitter for a while and he's liked a few of my things, so they're definitely looking at me."
Cullen, however, is not currently recognized as one of the top in-state football prospects for the class of 2019, with zero stars on his Rivals.com and 247Sports.com profiles. But, he might be one of the most intriguing prospects.
He is a left tackle and defensive end for the defending Class 3A state champions and has been impressive enough to earn scholarship offers from Air Force, Northern Colorado and Montana State.
"I'm grateful that I have three opportunities to play D-I college football," he said. "The opportunity to play, for me, is the biggest thing, and if more offers come, awesome. I'm going to consider every school that comes and gives me the opportunity play for them."
What makes Cullen intriguing is that he's grown so much physically in the past few months - and still has plenty of room to grow - that nobody has really seen him play to his potential.
Cullen has spent the past three years protecting the blind side of a quarterback - CU commit Ty Evans - that is quickly becoming a hot commodity around the country. In an offense that throws the ball a lot (677 passes by Evans in the past two seasons), Cullen hasn't given up a sack the past two years.
Cullen's highlight reel shows his potential as a blocker, but also as a play-making defensive end. Those highlights show Cullen at 210-215 pounds, however.
"All my past film and all my highlights, I don't think they really do me justice anymore, because I was playing at 215 and hadn't been working with (former CU star Matt McChesney at SixZero Strength & Fitness) so my technique, my size, my strength, my speed is all so much better," he said. "Just over the last few months, I've put on 25-30 pounds."
Even at 240, Cullen said, "I still feel skinny. I'm pretty sure I could easily hold 300, 315."
Because of his weight gain and potential for more, it's been tough for some schools to evaluate Cullen, and he understands that.
"A lot of coaches, I feel like, are confused on where to put me," he said. "That's a common theme is they don't know where I would play and where I would fit in a role on the team. I'm just trying to sell myself as an athlete."
Some schools have told Cullen they can build up his weight after he gets on campus. At CU, Cullen believes he'd need to have an entry weight of 260 or 270, if not 280.
"I think 250-260 would probably be the sweet spot for me and I'm trying," he said. "I'm eating a lot, working a lot. The weight that I'm at right now is definitely an acceptable weight and I've shown I can put weight on."
This summer is big for Cullen's future, as he looks to prove he can play at a bigger weight. He said he is planning to attend camps at CU, Nebraska, Oklahoma State, Utah and some Mountain West schools.
"I think every coach would want to see me (in person), especially with how much progress I've already made and how much more I can make over the summer," he said. "Camp is going to be not only a good thing for coaches to see, but also a good benchmark for where I'm at personally."
If all goes well, Cullen's hard work will result in more attention - and offers - from college coaches.
"It's a process and nobody's process is the same, and there's no written in stone set of rules," he said. "What happens happens.
"I just want an opportunity to play college football."
Contact staff writer Brian Howell at firstname.lastname@example.org or twitter.com/BrianHowell33.