Focus on efficient footwork
I am assuming you play singles more often. I have improved over the last few months from about your level to a strong 3.5. I don't feel as tired as I used to after playing matches. One reason I feel is because I am moving more efficiently now. I would recommend taking some classes from a local pro and tell him/her that you want to improve your footwork. Doing some shadow footwork drills on the court also helps. If you watch Federer, observe how he takes big steps and side shuffles most of the time. Also keep a low center of gravity by slightly bending your knees. The key is to not put your body in any awkward positions during any time of play.
Of course, footwork is just one part. Working on core fitness is another important factor.