I started riding again in my mid-50s. I was building my mileage up at an okay rate, but when I got up to about 18 miles, I couldn't imagine riding much further than that.

Then I started riding with groups. The first group ride was 25 miles; it felt much shorter than my solo rides of 28 miles. I wasn't going faster (at first), just farther. So, if there are biking groups available, I'd suggest you try some rides with them.

(Fair warning... if you start riding with groups, you're probably going to end up on a road bike.)