Surly,
Your plan looks pretty good to me. In facts, it looks better than good. You are doing the distance each and every week end. Good for you. You are also getting in aerobic workouts during the week in the spin classes. Nothing beats getting aerobic workouts in on a regular basis and nearly daily is best. That is what you are doing. Way to go.
I do not know how steep the hills you are climbing are. I tend to sit and spin on hills that are not too steep. If they are steep +7%, I will get out of the saddle. I can train on flat terrain for sittin and spinnin but I can not train for out of the saddle climbs on flat terrain.
If you are concerned about hills, you can try to simulate the climbs in your spin sessions. I am doing the Horrible Hundred here in central FL next month. I don't get over to Clermont at all so there are not any hills to ride where I am. I simulated the hills (which hit up to 15%) by getting out of the saddle on a spin bike and getting my heart rate up. It worked fine for last year's ride.
As far as I can tell, your prep should be more than adequate. I looked at it and was impressed.
I would think your biggest danger might be in being strong enough to hammer the first hills and beating yourself up too much. I say that because I did it once on a hilly century. I was in great shape. I was all excited. I attacked hills for 50 miles and after that it was climbing hills with toasty legs. So don't do as I did. Pace yourself.
Good luck to you,
Pat