I've been boxing for 11 years. I love it. I was trained to start with "bursts". I do a heavy 3 minutes, then light for about 10 minutes. Then, as I built strength and stamina, I up the burst times and shorten the light times. now, I'm doing a heavy 30 minutes or so with a only a short light time in between. My trainer said 30 minutes heavy is about the limit you want to go.

As your knee heals, you can add jump-roping to the cardio part. it's very good training.

I you have a YMCA, they usually have boxing classes or clubs. That would be a very good place to start, or at least talk with the trainers there for suggestions.