Sorry it has taken so long to answer.
I was over 50 when I rode my first century ride - Lake Tahoe with TNT. Done many since and now coach for the Wisconsin Chapter.
A typical 10 week program (mid-March through late May) looks like this: During the week, do at least 2 or 3 aerobic workouts of 30 to 50 minutes each, preferably on a spin bike or trainer (recumbant variety, if that is what you ride). Some moderately hard work during these workouts is good, but don't beat yourself up. On the first weekend, start with a long steady effort of 90 minutes (20 to 25 miles on the road). Extend your weekend workout each successive weekend by 20 minutes or so (say, 5 road milles). By late May, when you're up to 65 and 70 mile efforts, you will be ready. Long, steady efforts, these weekend rides, NOT races. For most people, not higher than 6 out of 10 on the "how hard am I peddling" scale.
Very important: Work with your coaches on hydration and nutrition, bike fit, cycling form and any soreness or other issue you encounter.
Last word, if you are lucky enough to have a group to ride with, do so. Don't miss those Team rides. Not only for the comradship, but to learn to ride safely and predictably with other cyclists.
GO TEAM!
Paul, a TNT Cycling Coach