To give you a sense of what you’re up against if you attempt to make it through P90X2, the sequel to the phenomenally popular P90X fitness program, here’s a sample exercise: the four medicine ball push-up. Yes, that means having a ball under each hand and foot. “It’s like doing push-ups during an earthquake,” says Tony Horton, the 53-year-old former stand-up comedian who created the original program eight years ago. That 90-day, 12-DVD regimen has sold 3.5 million copies, spawned an astounding number of before and after photos, persuaded politicians from both sides of the aisle to sweat it out...