I personally try not to think during my runs. I even make a concentrated effort to relax, stop thinking & clear my mind out of all sorts of thoughts, stop worrying about all world’s problems.
It is hard to achieve, happens rarely for me and only for 10K+ runs, but when it does, well, you just flow in a sheer bliss.

http://aeon.co/video/psychology/the-runners-a-short-film-about-deep-thoughts-while-running/