Unfortunately "bad knees" is at best a generic phrase which covers a wide spectrum of potential causes. Have your knees been assessed by a professional who gave you an recommendation as to what your problem is and what if any correction is possible? Without that type of information there is no legitimate response to your question.
It is called WORK-ing out for a reason.
I said getting fit was simple, I did not say it was easy.
Cardio burns calories, strength work burns fat.
Eat well to lose weight, exercise to get fit
You can not build a six pack using twelve packs
Often when we seek a magic bullet for fitness we end up shooting ourselves in the foot.
"I think calories are little germs in food that all moms are afraid of" Dennis the Menace