Hi Dawn,
I started on the static bike after ~3 weeks post-op, for weeks it was all about recovering mobility gently, and regaining a bit of strength, not really cycling in a "sports" sense. After 2 months or so I could do 30-40' on the static bike, but applying very low resistance (around 3 in a scale of 20, or 80-90 Watt).
Generally takes around 6 months for the neck of the femur to regain the strength/thickness from before surgery, so the risk of fracture with a fall goes back to 'normal'. I didn't want to risk and waited until 6 months to start cycling outdoors.
So I started 3 weeks ago, just a couple of gentle spins so far (today 35km/21miles, in gentle terrain), I could do probably 50km/30mi no problems, but prefer to see how it feels tomorrow. And so on.
Planning a 72 miles ride at the 4 months mark I'd say it's certainly possible, but ambitious.
Personally I opted for not setting in stone any sporting goals for the first year, and let the body guide me.
The thing with us sporty/competitive people is we are not very good at listening to our body, and we tend to 'hear' what we want ... I wish I listened to mine a bit more some years ago (not just because of the hips!).
You are doing well in your recovery, keep it up!