With gleaming shoulders, sculpted thighs, and washboard abs, your personal trainer sails through the gym, waving hello to clients, stopping beside a treadmill to adjust the speed for one, spotting the weight for another—all with an "I'm the kind of person you'd want around postapocalypse to restart the gene pool" confidence that you, sweating it out in your baggy sweats and T-shirt, feel you'll never have. In this age of Baywatch babes and buff Backstreet Boys, the Personal Trainer takes on a mythic quality. That's why it's easy sometimes to place your flabby body completely in…