Welcome aboard! Good to hear you are starting your recovery from knee replacement and the Doc said to go ride. I would leave any advice on how to train up to your physical therapist - if you have one. Given that there are no other issue I would think you should be able to do a 50 mile ride or even a metric by Sept. Have you discussed this goal with you care givers?
As an example - adding just 1 mile a day to your ride and riding 5 days/week should get you to 60 more miles in 12 weeks. Ofcourse you would not do it that way but I use that as an example to illustrate that it is not that big a stretch.