What do you think about?

Nearly every runner has heard this question at some point; I've heard it a few times, especially when talking to someone about running a marathon. I've come to expect the question when discussing running with a non-runner--it only makes sense to wonder what the brainis doing for all those hours. My typical, oh-so-very-clever answer was always, "Nothing. And everything." In a way, this is true, but I never really thought about it until a very recent run (about an hour ago, actually). The question entered my mind about one mile into the run, so I had seven more to ponder the answer and monitor my mind's behavior. Here is what I thought up (as close to my thought process as I can get and still be coherent while eliminating unrelated thoughts):
What do I think of
Nothing. Consciously, I think about nothing except the movement of my feet. During the first part of the run, I think about how I feel, noticing any tightness in my legs or any other soreness. I might also notice the weather and road conditions, but only very briefly. As the run progresses, I continue to monitor my body, but eventually it fades from the conscious to the unconscious, which I trust to let me know if anything goes wrong. Soon, everything is sent to the realm of the unconscious, and I am vaguely aware of the wind in my face and my feet hitting the ground. This is when I think of nothing. Then...
Everything. Without warning, a random thought pops into my head--it could be something that's been bothering me, some random memory, or just an idea to toy with. Sometimes my brain decides to start doing math problems, from the simple--what is 5x5?--to the slightly complex--what pace do I need to run X miles to finish in Y hours and Z minutes?--to the even more complex--how much work am I doing? These thoughts all come without conscious bidding, and sometimes I'm only partly aware that they're even there for quite some time.

I think I might change my answer to, "Nothing. And anything," to make it more truthful because I can't really think of everything.