Noonievut

I've been cycling for close to 20 years, mainly road, and in the last few years some gravel riding (mixed surface). I've also done some credit card style touring for multiple days (and still do).

When I was younger and had more time, I did many rides over 100k and a few over 150k. But usually I ride about 4x a week and my long ride is just under 100k on the weekend. My long ride is usually with a friend, who is reasonably fast and I do work a little harder to keep up (but feel fine afterwards, so all good).

I've always been intrigued by randonneuring, and thought this would be a good group to pose this question to (I could have posted this in the touring forum, but I'm less interested in multi-day).

My question is whether you can translate faster speed at a given distance (say 100k) to slower but longer rides?

For example if I avg 28k/hr on my fast ride (yes I know that isn't very fast!), so less than four hours, but I wanted to ride 125-150k and really take my time, going slower and stopping more, is it reasonable to say that I would feel the same during/after and could get through that distance pretty easily?

I've had my road bike for over a dozen years (custom, steel) and it still rides like a dream. I have a saddle I really like and usually am only a little sore in the sit bones areas after a long ride. My upper body also feels good generally, unless very windy or I'm working hard to keep up with a friend (using my upper body more).

I appreciate your viewpoints - thanks!