It's one of those days when the last thing in the world you want to do is go out in the cold drizzle for that hilly 4-mile run. What you want to do instead is stay in, eat your lunch, piss about on Youtube and Facebook, maybe even answer one of the 200 unread emails sitting in your inbox...
But you go out and run anyway. You get changed, align your body, loosen your shoulders, raise your head, find that inner drumbeat, and you run.
These are the days that test you; maybe they even help define you. You come back feeling better, stronger than you were before. These are the days that matter, because for a brief moment you get to feel like a warrior.
Later, you go home and apply the same attitude to your writing, and you get a sentence, a paragraph, perhaps even a whole damn page that sings. And it makes up for all the others that didn't.