Returning to training after injury

Recently I suffered a minor injury.  I went through the whole visit the doctor get an MRI and had him tell me I had an inflamed band of scar tissue in my knee.  It cost me about 1000 dollars for him to tell me to just rest.  For the entire month or two that I was resting, I was trying to stay somewhat active, but I just couldn't find anything that didn't hurt my knee.

About two months went by and I finally made my return to the gym.  Our sessions are about 2 hours long.  We have about 15 minute warm ups.  Then we drill techniques for about an hour to an hour and 15 minutes.  Then we roll for the rest of the class.  Now I was in damn good shape before the injury .  I was 190lbs at 6' and 37 years old.  I made my return on Monday for a gi class.  We actually had a pretty light day.  I was able to hold my own and not really gas.

I had some stuff going on with work this week so didn't get back in to train until Friday.  We had a nogi class and I have to say by the time we were done drilling I was gassed completely.  I found myself playing the dead fish through my rolls with the higher ranks.  At one point I thought I was going to throw up.  I know it was my first hard day back, but man was it discouraging.  I have never felt so out of shape while rolling.  

I guess my suggestion is that if you have an injury, really try to keep as active as you can.  Keep your weight down.  If there are cardio exercises you can do that wont hurt your injury do them.  I suggest swimming for this.  I know it's wintertime and you may not be in an area with warm weather, but there usually are inddor pools available to people.  swimming is the lowest impact exercise you can do, and it is so good for you.  It works every muscle in your body.  Keep your cardio up people so you do not have to play dead fish.  Keep yourself in the bjj mindset.  If you can do bjj specific exercises do them.  Watch some videos.  Keep