You already know how important fitness and flexibility are to hitting the golf ball far. Stretch, do pushups & sit ups, ride a bike. See a pilates instructor. They build flexibility and strength while retraining the body to work more efficiently.
Watch how smoothly the pros transition, even when hitting it far. Be patient at the top of your backswing, and let your driver do all the work. Take practice swings and listen. The woosh sound should be at the ball or later.
Jack Nicholas said that when he really had to hit the ball, he took the club back more slowly.