I'm a big believer in moderate exercise both pre and post op. I was in a lot of pain preop but managed a 20 minute exercise routine in the hot tub twice a day in the months before I had surgery. My physical therapist helped me design it based on what i could do;biking, various stretches, leg lifts, etc.
I had my surgery on a Friday afternoon, went home Saturday before noon and had my first outpatient physical therapy first thing Monday morning. It was fine. I wasn't pushed excessively. I enjoyed my sessions and felt looser afterwards. I did PT 3 times a week for a month, then continued exercising on my own. I was pedaling an exercise bike 10 days post surgery. I attribute that to all the presurgery biking in the hot tub. You have to know when to quit and how to listen to your body but I believe PT moved me along a bit faster. I'm nearly 9 months out and pretty much totally recovered and capable of walking all day without pain.
I am sure some people are going to need a slower pace than others due to prior damage to knee, lack of muscle from long disability, and more than one health issue. Having fibro, a bad hip, or excessive back pain can complicate recovery. Simply being determined to recover does not always equate with a quick recovery. That being said, go for it. Do as much as you reasonably can. Push to the point of discomfort, keep doing it every day until it gets easy, then do more. Stop if it swells or causes a lot of pain, either at the time or later so you cant sleep.