a bad run for me is one which I execute poorly. i run too fast. fuel too little, or too much. over train in the days leading up to it or even the day before. and i run out of steam before it's over. generally i finish the run feeling disappointed in myself like I could have gotten more from the run if I had just been patient or ran smarter. Even if by the end of the run I'm proud of myself for gritting my teeth and finishing, later on I will feel that I wasted the potential of it.
The reason that these bad runs can be good is because you have to dig so deep to make up for your mistakes. You have to stare down that wall when it smacks you in the face. And coming through that gives you a kind of confidence. Not an arrogance at all. Just a humble confidence that says, I have re defined my pain. I approached my threshold and now I know that I can dig beyond it and survive. Each time I do I push it farther and farther away. And that makes me stronger.