I suppose the fact that it took me so long to figure this out--precisely, it took until last Sunday afternoon--indicates that I haven't spent enough time kneeling thus far in my life:
Duh. The secret to kneeling for long periods isn't the amount of cushion for one's knees--it's where one's feet are. I can more comfortably kneel on concrete for an hour if I'm on a step or something so that lower legs are straight and my feet are still below my kneecaps, than I can with a huge wad of cushion on a flat floor when my lower legs go back up to make room for my feet.
No, I obviously haven't spent enough time kneeling.