As a fellow diabetic, let me add...be sure to carry your meter and a glucose source with you just in case. I don't know if you are a T1 or T2, but either way, you need to be prepared to treat a low should it occur.

As for training...I'd suggest building a good base first before worrying much about speed. Just ride, ride often, and ride long distances at least 1x/wk. When I say long distances, work your way up to a century, but don't increase your week over week mileage more then about 10%....so, your at 20 miles or so now. In a week, your long distance ride could be 25 miles. The next week, 27.5 miles, etc.