I'd start riding a road bike. It will help your recovery time immensely. A road bike is going to provide far better results than using a bike at the gym, as you're going to build up your core muscles substantially faster.
I can't agree with a physician who recommends weight lifting for this type of injury. The range of movement is just wrong. Pool therapy is also great, but there's nothing that compares to riding a bicycle.
Not only will your knee be stronger, but your entire aerobic system will benefit.
I'd start out at relatively high cadence (High RPM around 75-90 per min), in a high gear, so that you're putting minimal stress on your joint. The gained fitness can literally take seconds off your lap times.
Almost every GP rider and most Formula 1 drivers train on road bikes. As well, it's much easier to loose weight from your body than from your bike (if it's in race form).
Who knows, you might get hooked into a truly expensive racing sport,
Get well soon man. Glad to hear it wasn't worse. You'll be back to 100% in no time.
@s-eins I think the above (cycling) will help you tremendously. Even if you go out twice per week, you'll notice a vast improvement.