Somewhat surprisingly to me, my cadence is returning to the mid-80s range over 15 miles.
What made the difference? Switching to clipless and a seat positioning adjustment are the two biggies, I guess.

The other thing is that it seems that with the shorter rides I was taking, I wasn't really getting loosened up. I finally pushed thru the discomfort to 10 mile plus rides and found that doing 15-20 was actually relatively easy (other than dealing with Southern Louisiana temps and humidity). After 5-10 miles, I'm able to find a "riding zone" where a 75+ cadence just feels right and my average cadence over an entire ride has begun to climb back up and stay there over longer distances.

Strange in a way, but not so much in another.

I'll just accept it and move on.