I agree with both umd and tibikefor2. First thing is distance. Map out a training program that has you doing 200-250 miles/week in weeks STP-2 and STP-3. Include small training cycles, meaning increase mileage for 3 weeks at no more than 10%/week, then drop back for a recovery week and start over again with the increases. So it's a climbing sawtooth pattern, starting now.
Include one hard group ride/week. On that ride, starting now, do LT intervals, meaning get your HR up to LT or a few beats below, and hold it there for 15 minutes. Do no more than three of these. Maybe start with just one. Try to find out what the longest climb will be on your group ride, and try to stay with the leaders on that climb. You probably won't be able to, so cap it at LT. No higher. Gradually increase the length of that group ride until you are doing centuries about every couple of weeks toward the end. You won't need to ride more than a century on any single ride, as long as that ride has 5000' or so of climbing in it. It would be a good idea to do longer rides, but 100 miles is all that I've found necessary.
In May your fitness should be good enough for you to do two or three LT intervals midweek, also. Then you might try going over LT for 5-10 minute periods on your group ride. Do no more than two of these. That's really all you need to do. You can fancy it up with all sorts of fun intervals, but like umd said, don't let the intervals take away from your mileage. If you feel yourself getting tired, cut back on the intervals and group ride intensity until you start feeling better. Especially, over LT intervals can really sap you.
Ride in the mountains some in late June and early July. That will be really helpful.
I use a Polar 720i and download the information into my computer, so I can keep track of time-in-zone for the week. That helps me keep from overdoing it. No more than an hour of Zone 4 (LT) work and no more than 20 minutes of over LT work, per week, is necessary or probably a good idea between now and July.
Consistency is everything. Keep it up, week after week. Don't blow off your planned rides. Missing three days in a row can set you back three weeks. Don't try to "make up" missed rides, either. You will have to miss some workouts. Just continue with the plan as though nothing had happened.
See you there.