When I was training for marathons, quite a few runs were not physically satisfying (when you're consistently putting in 15 to 20 miles a day, your ass tends to drag on the ground like a hyena's much of the time) but I always enjoyed the psychological boost of engaging in heavy-duty goal-oriented behavior.
Now that I am a de facto jogger, and one who probably looks like a chimera of Bill the Cat and Alfred E. Neuman out there, this arrangement has been flip-flopped. It's easy to get a physical rush from an hour-long trot because that constitutes a long run now, but I can't help but inwardly mock my own efforts -- "If I am going to change my clothes and head out into the cold for a half and hour when I'm not even planning to race, why the hell do I bother at all?"
I know better, though. I run for the same essential reason I take certain vitamins and other pills and otherwise tend to the more elusive elements of personal well-being: If I think I might be restless and discontent now, just wait and see what happens when I sit on my ass for too long.