George Vafiades and Mark Merchant’s T-shirts read “Take the stairs.” But they’re not talking about sneaking in a few calorie-burning flights at your office instead of taking the elevator. Rather, they want you to bust your butt at As One Fitness, the new workout method they created in a 12-story building in Columbus Circle. Guess what’s part of the program?

George Vafiades

In a market flooded with boutique barre and spinning studios, Vafiades and Merchant, both athletes who share an extensive list of fitness training credentials, have carved out a niche for As One.

The workout could be a relative of the city’s best boot camps—or CrossFit, consisting of hard-core exercises delivered with individual attention and a firm footing in exercise science. In fact, As One is so intense that the founders recommend doing it only twice a week.

For class that’s so tough, it didn’t feel intimidating—even for those of us who usually stick to yoga. Instead of a meeting of meatheads, my class of five was nearly all women, many long and lithe and looking for a challenge. “Everyone works at their max, says Vaifiades. “And that’s a different amplitude for each person.”

That’s a nice way of saying, Everyone suffers.

The killer workout is based on interval training and variations on simple exercises with a mix of heavy props. We hoisted sandbags during squats and lunges, interspersed with a million burpees (where you get down in a squat and jump back into push-up position), and then hit the stairs. After four cycles, we switched to 20-second intervals of pull-ups, ropes, and more.

Mark Merchant

When two thirds through the class I started to struggle with my billionth burpee, Vafiades immediately noticed and offered me an adjustment. “Form is more important than repetition,” he explained. My trust in their guidance allowed me to push myself even further.

As One was agony, and—well—fun! Pain builds classroom comradery, I guess. And for two days after I could barely move. But I also couldn’t resist telling everyone I knew how many flights of stairs I had conquered. (Ten flights four times.) —Lisa Elaine Held

As One Fitness, 1841 Broadway, at Columbus Circle, Suite 713, www.As1effect.com, $25-$35 per class, depending on package purchased