My first experience at boot camp happened to coincide with skills-testing day, the last class of a five-week session at Altitude Peak Fitness. Toned boot campers lined the mats of the renovated warehouse-turned-gym, eager to discover how their fitness had improved from day one.

I’d been keeping in shape by running and practicing yoga, but I hadn’t been partaking in intense daily calisthenics like the rest of the class. I bit my lip nervously as I listened to the impossibly fit instructor explain the test.

We were paired off, given stopwatches, and told to time our partners as they completed a circuit consisting of: 50 squats followed by a 0.2-mile sprint, 50 push-ups followed by a 0.2-mile sprint, 10 pull-ups followed by a 0.2-mile sprint, 50 sit-ups followed by a 0.2-mile sprint, and, finally, a monster-truck-tire lift-and-toss challenge—followed by one last 0.2-mile sprint.

So, how’d I stack up? Let’s just say you could tell I was a first timer. As everyone else measured their progress, I struggled through the circuit, wishing I had devoted more time at home to strength training. (Ten pull-ups are harder than they sound.) As I worked muscles I hadn’t felt in years to total fatigue, the energetic trainer cheered me on. It felt good to push myself that hard—even if it meant I wouldn’t be able to climb stairs for a week without wincing.

I ended up finishing, albeit slowly, and though I was exhausted, I left wanting to see what I could accomplish with regular practice. The pull-ups could only get easier, I thought. And if I walked away looking like the trainer, it’d be worth every ounce of pain.

Altitude Peak Fitness boot camps mix indoor and outdoor instructor-facilitated workouts—like a run on the Cherry Creek trail and a power circuit at the gym—tailored for each participant’s fitness level and goals. The back-to-basics combination of cardio, stretching, strength training, and core work ensures you get full-body results.

Five-week sessions at Altitude Peak Fitness will run you $180-$220, depending on the program and class time you choose. But, here’s the deal: Every Saturday in LoDo, the 9:30 a.m. drop-in class is only $5. So get out your gym shorts, fill up your water bottle, and head to boot camp this weekend. See how many pull-ups you can do.