What makes it hard?

Last Wednesday, my dad and I were talking about running. He had just gotten back from a difficult run and couldn’t figure out what had made the outing that day so much harder than two days before.

My dad pulls out his running shoes three days a week and usually listens to the same playlist on his iPod and runs the same three-mile course through the neighborhood. He has noticed that on some days he is much farther along on the trail when a certain song starts to play than he is on others. On this particular day, he was moving much slower than usual.

On that same Wednesday, my roommate and I had our own terrible run. We turned in an awful 11:55 minute mile pace on our two-mile trip while, just the day before, she and I ran 3.5 miles along the same road and finished with a much more respectable pace of 9:30.

So the question my dad and I could not come to a definite conclusion on is, what makes one run so much more difficult than another, especially when the course and running buddy and playlist are all the same? And secondly, how can it be avoided?

I am interested in hearing your stories and running techniques so I can employ them and help myself become a better, stronger runner. Along with the questions I asked above, where does your motivation to run come from? How do you push yourself on a day when your run is more challenging than usual?