It's 4:30 p.m., and you've had one of those days at the office. You know you need to go work out. You also know you could go straight home and work out the cushions of your oversized, overstuffed sofa. Then you remember that he'll be at the gym. You suit up. He's John Dalton, a fitness instructor who teaches step, kickboxing, and weight-training aerobics classes at the downtown Y, and he's there everyday. This guy takes it to another level: He sings, whistles and whoops his way through the routines, sounding more like the Music Man than a drill instructor. You push yourself to the max and then some, because it actually starts to feel good. By the end of class, you've got a smile on your face, and you don't want to miss the next one. If you do, he'll wonder where you've been.