Your movement on the tennis court is one of the most important tools you can use to win the match. There are many different ways to do this.
First, by running after every ball and getting many of your opponent's best shots back in play, you can force them to go bigger and try more risky shots. This will often produce a lot more unforced errors as well as frustrating the other player.
Another way to use your legs during the match is to challenge yourself to get in position and hit the ball at, or just before your opponent's shot gets to the peak of the bounce. This will take time away from them and once again, make them feel like their shots need to be even better. You can also use your movement to recover after your shot and cover the opening on the court. This will make the opponent feel like there is little or no opening to hit their next ball to, and force them to come up with a more complicated and difficult shot.
My favorite way to use your legs as a weapon is to move forward during the point and apply pressure to the other player, forcing him/her to hit a passing shot or lob. Something as simple as moving around as your opponent is about to serve or taking a step towards the expected direction of their next shot can really affect their ability to hit it well.
Make sure you keep your movement as one of the key reminders during the match and use your legs as a weapon to put pressure on your opponent.