Okay, everybody. This has been hugely helpful. Thank you all so much.
Someone asked how long this has been an issue and it got me thinking. I used to rarely ride more than 35 miles at a time until, I don't know, maybe 4 weeks ago? A month and a-half maybe. Probably. But I lengthened my 'long' ride to maybe 50-80 miles on the weekends. I just figured, hey, I'm in fairly decent shape. I can climb and ride for a couple hours without issue. What's another hour or two? Those long rides are comfortable for me, I am not sore during the ride. But I am tired at the end, which I almost never am if I've only ridden like, 35-40 miles, say. Those long rides, I feel them in terms of fatigue right away.
The other issue is without question protein. Did not even consider it. I am woefully short on protein on my almost-vegan diet. For some reason, I not long ago (6 months maybe?) started becoming grossed out by animal protein. Totally unappetizing. Yuk. Rancher's grandson from West Texas. He is rolling over. But I did nothing to supplement that lost protein, because I am an idiot. So, today I spent an inordinate amount of money on various protein sources and will get to them at 1-2 gm/kg/day, as advised.
In younger years, I ran a few marathons and ultras. No talent, just persistent and determined. Anyway, glad some folks brought up recovery activities because I always had recovery walking and runs in my plans during those marathon days. I really think it helped me. Over-training even for ultra runners is a real thing, IMO. I now live in a highly urban locale and ride my bike everywhere. But it is totally walkable, so I can switch that around no problem.
So, I've got my to-do list, thank you all very much. I'll update this thread when there is something new to report. Thanks again, folks--