Same for me, I try to make it meditative in a way. I don't use earbuds or anything, just try to continue existing and moving. Sometimes identify birds. Practice talking to myself in languages I'm studying. In races it turns into body state monitoring and trying to determine when/if to adjust pace. Check in on form. On heart rate. Pick someone up ahead to try to catch. Count down to the next gel or electrolyte tab or water station.

I've done quite a few 2hr+ runs (half marathon to marathon) and 100 mile bike rides, and after a while for me it turns into a psychological game.

Trail running, if you haven't tried it, is much more "stimulating" I might say, depending on where you are there's a lot more focus and attention required to stay on the trail, to stay upright (slipping on mud, ending up in a river), to dodge trees and rocks as required, etc. I personally find there's much more of an aspect of being "in the zone" for trail running, and especially in races, when I miss a turn and have to stop and backtrack, it becomes really obvious that I was in some kind of "flow" state and then got pulled out of it.