Updated: July 25, 2013

Today's blogger: Marissa Stephenson, SHAPE senior associate fitness editor

Have you ever felt secretly cocky about your fitness level? One week ago, that was me. I'd been logging regular 40-minute runs, lifting weights, and doing my best to walk everywhere-including home from work, which at four miles is no short trek. So when I signed up for an Intense Core/Cardio class at 360Fit, I wasn't too worried about the "intense" part. Class instructor and founder of the new gym, Shanna Farrar, told me the session would be 60 minutes of nonstop cardio blasts and strength exercises, and that I could take breaks if I needed. I just smiled and shook my head.

I then proceeded to shake, sweat, and suck serious wind through one of the toughest classes I've ever taken. Not only were the moves challenging on their own-the workout included burpees, squats, push-ups, and bicycles-but we had no breaks (besides a second or two to gulp water, that is). In between the exercise sets, we ran up and down stairs or did jumping jacks and knee lifts. By the hour's end, I could barely sink into a squat without my legs wobbling and my tank top was soaked through. My realization: Just because you can easily do the same workouts you always do doesn't mean you're in tip-top shape. And shaking up your routine with classes that challenge you to do more (my classmates and I agreed that we would never push ourselves this hard on our own!) is key to truly becoming the fittest you can be.

If you can do your cardio workout on autopilot, click here for challenging new interval plans.