I treat it as time to concentrate on me. I tend to put the world to rights. Rehearse arguments, revisit decisions. Talk to my dad (in my head, he's dead, so can't actually talk to him) Generally work through all the worries and bothers that otherwise tend to clutter up my head. Somehow the problems get left out on the hedgerows and I feel better for the experience both physically and mentally.
That sounds really heavy, it doesn't feel like it at the time. it isn't necessarily the whole run either, there's things like "I wonder what the bird over there is, haven't got the specs, so can't see it clearly, wonder if it will sit there long enought for me to get it in focus... oh it's gone." and "ooops, that was slippy" or "urgh, just stuck my hand in the mud my trainers left on the stile, wonder if i could wipe it on my T-shirt - nope, that hasn't helped." And so on. Sometimes out loud. to myself.
Maybe therapy would be the answer???