I agree with IGottaRun - I am usually on autopilot, just trying to psyche myself into continuing to run, and then being overjoyed that I actually am STILL RUNNING at mile 8,9,10 etc!!
I try to think about things like plans for the day, or even do some praying but I find that my mind just continually slips into a fog where there aren't really many active thoughts going on besides looking at the time/distance on my watch! It's weird, but I agree, that my mind actually LOVES the break b/c I am the kind of person who's head is usually WHIRLING with 14 different ideas at the same time... so I can use a little downtime - in fact, it's probably what I love most about running is it gives me permission to take a little MENTAL break, while I do a little PHYSICAL exercise...
I think I'll always wish that my mind was more productive, but... that's just the way I'm made I guess! 