Right-O! The process of increasing endurance does seem to take a lot of time, years. It's been noticed that older riders have more endurance (up to a point), and I think that's just time-in-grade. Some body modifications are slower than others.

I didn't figure this out. It just happened to me and I noticed that I was getting better results than seemed sensible. I knew it wasn't talent, so it had to be training. I have (had) a competitive streak plus I just enjoy being at my limit, which involved finding said limits. I'll never forget the pain level on my first 300k, 9000' of steep stuff. Some rando planners (I forget the words of course) love to put in some 18-24% stuff about 3/4 through the ride. No walking or paperboy, this is a timed ride!