I remember my first circuit class at the gym with Tom. Well, technically, my second one—but that first real attempt where I made it through the full warm-up and thought I might actually die. I’d done my fair share of running, even dabbled in weight training, but five minutes into this class I was puffing like I’d never moved in my life. By the end of the 45 minutes, I was drenched, beet-red, and wrecked. I didn’t go back for a month. I didn’t like the pain.