Almost any exercise is good exercise.
That said, some people waste time on routines that aren't best for reaching their goals, trainers say.
Here are general tips from Shaun Knight, a certified athletic trainer in Virginia Beach, Va.:
* Think about variety, not just time. Keeping your body guessing is the best way to torch calories. "If you ride an exercise bike at the same level for an hour, it may not be beneficial -- especially given the time you're putting in," Knight says. "Your body will get used to it and start to think it's easy."