I have made a very interesting personal discovery about bad runs. Back when I ran less often I usually attributed bad runs to fatigue. If I had a bad run I would take a day or two off. Now that I run a lot more often I wouldn't even consider taking a day off unless I have had several crappy days in a row. Often I find, I can have a horrible run, then run harder and/or longer the next day(or sometimes even later the same day) and have a great run.
What a strange mystery crappy runs are!
Have found the same thing. Sometimes a crappy run (sometimes two in a row) but then miraculously the next day an easy run is actually at a much faster pace than I thought it would be, or I can really crank out a great interval set.