Pain

In the interest of full disclosure, here is something every runner/aspiring runner should know.  It hurts.  If you are running hard, it will hurt.  Not injury hurt.  But I-am-pushing-myself-to-the-limit hurt.  The kind of hurt that fades quickly in your memory to merely "hard effort" so that our brain tricks us into running again.  The kind of hurt that is easily over-ridden by the joy of personal bests and conquering difficult things.  It hurts, but fortunately the pain is temporary.  

G. Gordon Liddy of Watergate renown famously held his hand in a flame to show his pain tolerance.  When asked what the trick was, he replied, "The trick is not minding." (Quoted in The Quotable Runner). This is the trick to enduring long distance running as well.  We train our bodies to not mind the pain.  We can train to reduce the amount of pain we feel, or how early it comes to us.  We don't train to avoid pain. Rather, we train not to mind, because we have experienced pain before, liived through it, and become stronger for it.  It shouldn't hurt the whole time you run, or every time you run, but if you expect (and meet, and conquer) a little difficulty once in a while, you will be stronger for it.  

Now that I think about it, I guess the same thing can be said for life, too.