This exactly. I can roll easily at 15 mph for miles on end on relative level path (except for my knees obviously) and that appears to be where I naturally end up and enjoy riding. I don't feel like I am "racing" or pushing anything, it is comfortable mentally and physically. Every time I force myself to slow down, I ended up right back at 15 over and over. I listen to my body but my knees don't say anything until it's too late which throws off my judgment. What does slow me down now are things that provide immediate feedback like overall lack of energy, legs burning, high HR (I seem comfortable maintaining 130-140) but unless there is a hill or I didn't eat right, those usually don't happen. I'll have to learn and get used to a sustainable touring mode and I now know me knees ARE my limit. On this last trip I tried hanging far enough behind people without being obvious I was pacing with them but eventually zoned out and got too close and it was awkward.