“Let’s do things a little differently today,” my trainer remarks as he picks up the 25lb. dumbbells for my squats.  “Try to get at least 15.” “Okay,” I’m thinking. I’ve been using the 20lb ones for several years, how much heavier can these be? A lot, I discover. By the tenth rep I’m wondering if I can keep coming back out of the squat position and my heart is beating harder. I finish the 15 reps, though. We follow the squats with 15 pullups and 15 leg raises on “the chair.” Knowing I will repeat this sequence twice more makes me a little more tired, but I persevere. That’s what I’m here for, I tell myself. “Instead of reps on this next set, let’s…