I don't get it. You went from a painful 75 miler to a comfortable 20 miler and give the credit to the bike fit. I will add that riding consistently over the last 23 years, if I slack off on my mileage, the shorter ride speeds tend to fade. If I increase my distance, the shorter rides tend to get faster. MOF, recent years, I topped my faster speeds on local rides of 10 years ago by pretty good gains and still have had no bike fits.
Either way, the things you mentioned are things I would have adjusted the second I started feeling any discomfort on the 75 miler. Other than shim on the pedal, all seems pretty basic. I'm guessing with 11 different bikes, no discomfort on 35 centuries, I must have saved myself at least by paying attention to my body. $1650
I was training to do a 140 mile ride and the 20 mile ride was just my first ride. Secondly, it's very difficult for someone to fit themselves because you change one thing, it affects the other touchpoints. Thirdly, if you are having pain on the bike, chances are you aren't going to be able to fix that on your own without a bike fit.