Depending on how much you love tennis and how long you want to play I would never make running a part of my weekly cardio. Better to stick with lifecycle, eliptical, rope jumping, ect. that are less jarring on your joints. Also, weight lifting and plenty of stretching can really help your game. Although interval sprints are okay to do at times, for maximum time reward a far better workout for tennis and fitness is to do a combination speed/plyometrics workout 2-3/wk lasting about 30-50 minutes. I create my own workout using cones, jump rope, boxes, ladder, medicine ball, crazy ball, tennis racquet ect. in an aerobic studio with mirrors and music. Great variety,fun,tremendous calorie burner-my shirt is drenched, and gives you a huge competitive edge against your competition. The pros do this every week, but very few adult tennis players do. This workout is so good and fun, I would do this even if I wasn't a tennis player just for the cardio and calories burned.