I'm always amazed by the little mental gyration required at the beginning of a two-hour out-and-back run. If I thought about that fact that I'm going to be moving like that for two hours, I'd never start. But I remind myself that I like the processional quality of the run--the notching of each mile, the pause at 5 miles to get some cached water and check my average pace and HR, and the pause again at my turnaround point (variously 6 1/2, 7, and 7 1/2). The return leg is entirely different than the out leg. Sometimes I'm tireless, pushing hard and feeling great, then holding pace through the end. In the summer, it's often more of a true endurance run. A huge feeling satisfaction when I crest over and down the final small hill and pull up short, then walk back to the car. Nobody just watching me would have the slighest idea of how much satisfaction I'm feeling--regardless of whether I'm wiped out or just exhilarated and tired.
But I'm always amazed by how easily the mind is willing to retreat from 15 back to 10 miles, so that, when I'm not training well, 10 miles seems like a really long run. But when I'm running those 15s every Sunday, 10 is just a solid midweek run.
The mind is a party to all this.
Am I the only one who finds it almost impossible to run 15 miles in a series of repeat loops in a park? I ran 15 once around a 1.4 mile park loop, and it was the worst mental grind I can remember. Horrible! I think it cuts back to our primal origins as endurance predators. Our souls know that it's just make-work, nothing that will culminate in a kill. The long out-and-back runs, though.......there's always hope.