Three weeks. That's all the time you need to get in shape. It doesn't matter whether you haven't worked out in months or you're a gym rat looking to take your fitness to the next level. Follow our program and you'll emerge 21 days later stronger, faster, and more flexible — and looking and feeling better, too. We tapped some of the country's leading strength-and-conditioning experts to create an ideal week of training, a seven-day template that repeats three times, gradually increasing in difficulty each week. All these workouts are scalable, and . . .