"moderate pace of 18-20 MPH in past years"
"had trouble hanging onto a couple of 30 somethings at 21 MPH"

I'm 49. I've done one ride where I averaged over 18 mph. It's not uncommon to ride that fast if there's no wind or hills or heat, but not as a day-in day-out practice, and I'm generally not going to hang with anyone going 21 mph unless I have a stiff tailwind.

That being said, I just went over 4,000 miles for the year, and will probably hit 7,000 by New Years. So based on my experience, your training is not in line with your expectations. (Unless you're also doing lots of non-riding exercise).