Running difficulty? It's mostly in your head

I reached a point in both my runs this weekend where I thought "Gee, this is hard." But when I thought about it I couldn't really identify the thing that was making my run feel difficult. I wasn't having trouble with my breathing or my knee. My legs were beginning to lose energy but they weren't painful. I realized that the run felt hard because I had covered a certain distance and assumed that's how I should be feeling.

The mild discomfort I felt after four miles of steady pacing was nothing compared to the "I just want it to stop!!" feeling I'd experienced during last Sunday's half marathon. I tried to think about how I'd felt four miles into that 13.1 mile race. Four miles represented only 30% of the distance I'd prepared to cover, while on Sunday it represented my full distance. Had I previously decided to run five miles instead of four would my discomfort have started later? It's clear to me that the hardest part of running (until you reach your physical limits) is preventing a perception of difficulty from undermining a good run.