Any and all exercise is beneficial to tennis, up to a point.
Worse is to lay on your deathbed, getting fed intravineously, and not moving.
Tune your workouts to your needs. Some people need quickness, some strength, some explosiveness, some endurance, we're all different and have differing needs.
For you specifically, I'd work on your first 3 steps, more explosivness first, then some endurance much latter.
Do boxing, basketball, karate, taichi, something that forces you to react quickly and decisively.