There's a pretty good post over at www.active.com about making the most of a bad run that is worth checking out.

We've all had a clunker of a run before. I used to be of the mind that I'd just barrel through it anyway.

But I've matured as a runner (at least that's what my more mature running friend keeps telling me), so, now, I consider the situation -- Have I run 5 days in a row? Am I planning to run tomorrow? Have I overdone it on classes at the Y?

I usually arrive at the conclusion that I just need a day off.

And, honestly, there's no shame in that. Sometimes your body just needs a break -- and it's better to take a day off than push through and make running a miserable experience.

That said...you have to be careful you aren't going too easy on yourself. The article I linked to has some good advice on that.