Alright, I'll try. I'm also 67, but I've been riding pretty seriously for the last six years and. By that, I mean 36 miles is our short ride, 52 is our longest regular ride, and we occasionally go 60+ miles. 100 miles a week is a bad week. A good week is 200+ miles.
We've been working on increasing our average speed on the shorter rides, especially. To complicate things, I live in a pretty hot and humid area. Not Texas or Florida hot, but the heat takes something out of you when humidity and temperature both approach 90. We mostly get home almost too exhausted. And we kind of like it. Our bodies tell us when we need to stop. I carry six bottles of water, my gf carries four. On long rides we drink it all, including some diluted Gatorade. We eat trail mix 18-30 miles into each ride. An after ride nap is your friend. Either that or a cup of strong coffee.
My advice is to ride more, rest more, and consult a professional Barber. Good luck.