Here are suggestions for getting the most training benefit from your tennis game:
Play singles, not doubles. You'll have to go after more balls and cover more territory.
Choose slow court surfaces. Points last longer on slower courts, so you do more running and hitting.
Be aggressive. Go after every shot, even if it's going out. Run, don't walk, back to your position.
Keep moving. Run in place or walk quickly in circles while your opponent is gathering balls or during other pauses.