A couple of questions/suggestions.

1. What do you eat before your ride? Are you eating and drinking enough during rides? As your rides get longer, refueling and hydration will become all the more important.

2. Do you mash the gears or spin wildly, or is your pedaling smooth and easy? That is, are you pedaling in the most efficient way for you or are you wasting energy?

3. Do you prefer riding with other cyclists or solo? For some people, the social aspect can be helpful.

4. Since you mention routes in your post, are you trying to tackle too hilly a route for your present conditioning? Since you will be extending the mileage so much, you might not want to also deal with serious hills--ie, face one additional challenge at a time.

5. How hard do you ride? Do you push yourself a bit to get stronger and faster? I like to see the scenery when I ride, but I found that when I just noodled along, I didn't get stronger. This doesn't mean that you have to get crazy-serious about training, but that if you push yourself a bit, you'll see the improvement in your strength and stamina.

Good luck, good health and happy birthday. There's a fellow around here who has done a century a month since he turned 70 about four-five years ago, with the exception of a couple of weather-enforced cancellations. So as you get older, your rides will get longer and longer and longer....