She wasn't afraid to admit it was killer.

Kate Upton is never one to shy away from a tough workout. She's gained a reputation for pushing around sleds loaded with 500 pounds and making 200-pound deadlifts look easy. (The model told us all about how she worked up to lifting heavy in her cover story this month.) For her latest challenge, she drifted from her usual gym sessions, participating in a grueling workout led by none other than the Marines.

The no-nonsense cardio circuit-which Upton participated in to support Marine Detroit Week and her fiancé Justin Verlander's Win for Warriors Foundation-included intervals of burpees, traveling push-ups, running, jump squats, and high knees.

Upton may be a total badass (with some seriously impressive skills), but she still found the nonstop, super-intense workout to be uh, just a little bit tricky. "Normally, I like to take a little rest in between sets, but to keep going was a very difficult thing," she told the Detroit News. (Talk about a boot-camp workout!) "The Marines put on a ruck of 100 pounds, and they're going for 20 miles. They never stop, so it showed me that I have a long way to go before I ever put a ruck on my back."

While she was clearly a great sport about being put to the test ("I have few more workouts to go before I'm ready," she wrote on Instagram with a cry-laugh face), you gotta give the girl props for challenging herself to a crazy-intense workout. After all, even for someone notoriously fit, switching up your fitness routine and trying something out of your comfort zone can be scary-but seriously rewarding.