I'll just put this out there. When I exert my legs on a long run, my brain seems to take a break.
Today, for instance, I set out for my long run without a clear goal in mind. But I was feeling pretty good after the first ten miles, so I decided that I would continue on until I got to 75% of the distance of my upcoming 50k. After several minutes of struggling with the math in my head, I finally calculated that 75% of 31 miles is 22.75 miles.
It wasn't until I was in the car on the way home that I realized my mathematical error.
And it's not just math. Several weeks ago, I arrived at my doorstep after a long run and came inside panting. After I announced to my wife how far I had run, she asked me what route I took. I responded that I had several times around "that long green area in the middle of town where all the people are walking." She looked at me quizzically. "Do you mean the mall?" "Yeah," I said, "the mall. That's what they call it. I couldn't remember."
I'd like to think that this is a normal, easily explainable thing, perhaps caused by all the blood being directed to my muscles rather than my head. But I have never noticed it talked about on blogs or in runners' magazines, so maybe it's just me.