Interesting to think about this.
In the decades of riding before I discovered randonneuring, I'd say 40 miles was my go-to distance. After two or three years of rando, 200k became the sweet spot, ezpz. That held true fur several years, even after i moved from Indiana to west Texas. Then i moved to Oregon; with elevation added to the mix, things changed. I've switched from recumbent to upright in no small part due to improved climbing speed. Even so, the sweet spot is probably more like 100k. I still ride a 200k or longer monthly, but that distance is now more of an event.