Well.... 35+ masters is a damn fast group, definitely faster than the cat-3s and just slightly slower than the 1/2/Pros. I'd recommend starting in cat-5. You're starting just at the end of the previous season, so you can get ready for the next with maximum amount of training time to get ready.

Average speeds really doesn't relate to bike-racing very well. There are guys who can average 28mph on time-trials who get dropped in crits all the time. Then there are guys who can barely average 22mph and they can win races. It's the range of speeds that you must be concerned with and it can vary from 20mph all the way up to 41-42mph during the race. If you can't hang on just one of those 40mph+ spurts, you're gonna get dropped.

The preparation plan can be set up in several phases roughly as follows with the most amount of time required in the beginning:

BASE TRAINING 2-3 months
For the next two months, you just want to get in a lot of miles. For the 1st month, start out with 20-30mile rides a couple times a week at steady pace you can hold the entire time. Get in one 30-40mile ride if you can on the weekend. If you go too fast and end up having to crawl home, slow down, it won't help you improve as fast as doing the entire ride at a steady pace. Try to get in enough rides for about 175-250 miles/week for the 1st month with roughly a 20-30mile total increase per week to end at 250 miles/week. Again, all at a steady average pace for the entire ride that you can maintain the whole time. These long rides can get boring, so work on pedaling-form, shifting-strategies, eating and drinking without disrupting your riding-position or having to take your eyes off the road.

This next month, you'll want to keep up the long mileage with steady weekly increases. Try to get one day at 50-60miles in (eat and drink lots on the ride) and end up at 300 miles/week. You'll also start some weight-training in the gym as well. About 1-2 days per week with light weights/high-reps (60-75% of max-lift and 10-15 reps). This builds up form and strength in your joints in preparation for higher weights next month.

STRENGTH/AEROBIC TRAINING - 2 months
With the base-mileage phase done in 2-3 months, you'll want to transition into strength-training in the gym and more aerobic workouts on the bike. Still do 1-2 days at the gym with higher weights/lower-reps (75-80% max-lift 5-10 reps). Add one day of heavy lifting if you can. On the bike, you'll want to back off on the mileage to only about 250-miles/week. You'll pick up the intensity a little with some hill-climbs (25-30 minute hills) to work your heart & lungs more. You can also add one day of hill-intervals up short hills of 0.5-1.5 miles. Still keep one day of an endurance ride of 50-60miles.

The 2nd month of this you'll want to increase the weights to one day of high-strength levels of 85-95% max-lift and 2-5 reps one day a week. A second day can be done at the previous level of 5-10 reps. In the last 2 weeks of this month, you'll want to transition to speed-work on the weights. Riding can also be cut back in mileage to 200-miles with higher intensity. Do two days of interval work, one day of hills and one day of flats. Still do one day of hill-climbing per week and one day of an endurance ride.

SPEEDWORK 1-2 months
Now we'll cut off all the weight-traning completely and work on building speed. Your average-speed will take a tremendous jump here, like from 18-mph to 23-mph in this phase as well your maximum-speed. We'll reduce mileage to 200-miles/week. Add one short 15-mile day with 3 all-out 100% screaming sprints. Go all out with 100% effort until you're all spent. Rest up completely 3-5 minutes, and do another one. After the 3rd one, go home. Add one more sprint to the sprint-day each week. You'll fine-tune the flat-intervals as well and structure it into sets of 2-2-2 minutes or 1-2-3-2-1 pyramid sets with plenty of rest in between. No more hillclimbs, but still do 1 day of hill-intervals and one endurance day.

The 2nd month of this phase, you'll cut back mileage to 150-180 miles/week (3-4 days) and pick up the intensity even more. You'll want to do about 8-10 sprints on your sprint-day. Increase the intervals to 2-2-2-2-2 sets or 1-2-3-5-3-2-1 pyramids. Cut back the endurance ride to 40-50 miles. Get a full day of rest between riding days.
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Initially, you may be looking at 15-18 hours of training/week which will get trimmed down to 12-15hours by the end. This should place you right into early March with a week or two of easy riding to get ready for the season-opener. During all of this training time, you want to do rides with local bike-clubs if you can. Aside from just the physical training, the other 50-75% of racing success is learning the mental strategies. How to draft, how to move around in a pack of riders, how to position yourself for sprints, how to corner and hold your line in a pack elbow-to-elbow with people, how to push/shove and counteract others pushing/shoving, how to pick out stronger riders in a pack and develop strategies to outwit them are all important components of the mental chess game. Hopefully, you'll get the physical-training down as well as a parallel track of the mental game. Then you'll be able to win your first cat-5 race outright.