I'd find it very difficult to do an entire ride (90mins or 2 hours is my everyday usual) glued to the saddle, even if I didn't ride SS and have a lot of hills where I am. It comes so naturally to me that I often find myself doing it without even thinking about it, just to stretch my legs, and because I enjoy it.

I got my first practice at it about 40 years ago as a kid, when my saddle broke and I was too cheap to buy a new one for a few months. Didn't do the same sort of hard riding back then, just a paper route and to school and back, but it must have been enough to acquire the knack.