I have rode the MS150 (Houston to Austin,TX) twice. Both times I felt great when I got off the bike. Last year I trained with an injured knee, so it can be done.I would listen to your doctors on how much you can do.

What I did both years was ride often (mostly 20 miles). We did two organized rides (max 76 miles) to get a feel for the group dynamics. I logged over 550 training miles in two months both years. It can be done, you have to be on the bike about 4 days a week. I would just build on each ride and try to increase the distance.