"I guess I'm not really at all sure what I'm asking. Perhaps looking for ideas on whether I should be doing more, or am I approaching over-training? Should I expect that I can do better, or should I be happy with what I've got?"

I'm 63, started riding again about 10 years ago. I was never a strong cyclist but was a pretty good distance runner in my day. Sounds like we ride in similar terrain, not many flats where I ride. My experience was that it took 4-5 years for my legs and lungs to become cycling strong. Sure, I could go for an occasional long ride of 60 miles or so would even do a century once a year but I knew that I was extending myself big time. After 3-4 years the yearly winter break of a few months no longer was debilatating and I could jump on the bike and ride 25-35 right off, albeit in moderate time. Then I can build on that for the rest of the riding season. This year I've lessened my daily mileage and try to ride harder for shorter distances and find that my climbing and cadence has really improved.

IMHO you're doing great, you might even be riding a bit much. Maybe do 15-20 at a brisker pace to build muscle rather then endurance and bank on that next year. Lots of people on this forum are riding 50 miles plus after a few months of biking and I just wonder if that's a good long term training plan. I'd advise to not get caught up with comparisons. BTW, I'm so jealous that you can still run. Can't take the pounding any more myself.