Southern California fitness entrepreneur Anthony Geisler knows a thing or two about boxing. Having previously run LA Boxing, UFC Gym, and Rumble Boxing—three iconic, boutique boxing brands—he’s recently observed an evolution in boxing as a fitness trend.

They say it takes 21 days to make something a habit. But what about 18 months? That’s how long I’ve been going to Rumble Boxing, and I think this is just a little more than a habit. As cliché as it sounds, it is actually a lifestyle. I’ve even owned ...