Several years ago, I was visiting my FIL in Northwestern Illinois. I took my bike with me and while on a ride I stopped in a small bike shop in a small town along my route. Above the counter was a sign that read "The more you ride, the better you get. The better you get, the more you ride." I think that just about covers a lot of the answers to questions posted on cycling forums.

Bike fit is essential for any type of riding, let alone long distance rides. But having a bike that is your size doesn't mean you have a bike that fits, especially when it comes to road bikes. If you're not dialed in to your bike, you can expect some sort of pain or discomfort somewhere. If you're not sure how to dial your bike so that it's optimal for you, have it done professionally. Wasting weeks or months trying to make adjustments that might make it worse isn't worth the savings of a professional fit. Also, having the perfect saddle for you is as important as a correct bike fit.

I don't know if I qualify to be called a long distance rider, but I'm a 10K+/yr rider, this month will be my 8th 1,000 mile month this year, I've done a several centuries (last one in May of this year), and I'll be doing a 220 mile cross state ride (my 4th) next weekend. I purchased a new bike in April and had a Retul fitting done and I experience no pain when riding and little to no discomfort on any of my rides. I recently turned 71.

So to answer your questions; "Is it a reasonable expectation for us, as we get older, to ride with absolutely no pain whatsoever?" and "Is it a reasonable expectation to be able to ride any distance, or until the "tank" runs out, without any pain whatsoever?" The answer is "Yes!"