When you feel too tired to hop on the treadmill for a half hour or to take an hour-long aerobics class after work, opt for just 15 minutes. You may find
that when the 15 minutes are up, you actually have the
energy to finish your workout strong. If you don't, you at least got a shortened version of your workout into your schedule and you can try a full workout the next day instead. If it works for your schedule, exercising in the morning has the advantage.