Your fitness program should include regular workouts with quality exercises. The length of the workout is not so important as what you do while you are working out. You should start at twenty minutes and gradually work your way up to an hour. You don't want to be exhausted when you work out or you will not get as much benefit from the exercises.
If you want to be a better tennis player, enhance your ability to change your focus quickly from far away to near by. This simulates the same focus it takes to hit a ball, and then react to a ball hit by your opponent. Soon you will be beating your opponent with ease!
When you make up your mind to become physically fit, do not forget to have a check-up prior to getting fully involved in vigorous exercise. It's particularly important to get professional medical advice if you face significant challenges on your way to getting fit. Even if you are already close to being fit, your doctor can help you with a little expert advice.