I'm a certified personal trainer and I absolutely love Insanity. I've done every kind of exercising and for tennis specific, I find plyometrics and interval training to be the best for me. I understand not every one will agree with me. This is what works for me. My knees feel better, I'm quicker on the court, my endurance goes way up, my foot work is way better.
The other thing that is great about Insanity is that it requires no equipment and it works your core. And the consistent stretching that is done with every routine will increase mobility and flexibility.
The great thing about it, if you can squelch the ego a bit, is to just do it at your own pace. Even if you can't do full squats or full on jumps at first, your body will eventually respond. It's a work out system that you can grow into if you know what I mean.
And of course, I'm a big believer in diet. Get plenty of Omega 3 oil. Cut down on the sugar, include extra fiber in your diet along with plenty of fruits and veggies and watch your fitness level steadily increase. Also beware of bad oils in your diet. We are inundated with crappy Omega 6 oils that actually cause inflammation in the body. Watch out for soybean oil, it's in so many processed foods.