Everything I see that's changed since the previous run.
Everything I see that's changed since the previous season.
Everything I can feel about my body moving
Everything that's happened that day.
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.