Knee injury and tennis elbow, need exercise ideas

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I've been working out for about a year and a half. My usual routine is walking the dog 40 minutes per day. Cardio for an hour 3x a week and 2 days a week 1 1/2 hours of cardio and 45 min of weight training. I started with a torn miniscus and osteo arthritus in the right knee, so I've tried to strengthen the major muscle groups in my butt and legs to stabalize my knees. Lately I have developed "tennis elbow" and it's getting worse. I went to the doctor and they told me to rest it for 3 weeks, no activity.

I've worked so hard to get exercise into my life and lose weight, but I am scared by only doing cardio I won't have enough exercise to keep losing or even maintain. I tried water exercise, but it hurts worse than the eliptical, walking or bike.

Any suggestions?