Some days are just like that! Last week I had a glorious
8 mile run. Slightly faster than usual, felt fantastic, legs felt light, etc etc. Yesterday, I had a horrible
9 miler. Legs like lead. Fighting for every step. Ugh.
Don't beat yourself up. Yes, it's good to know when your body just needs a nap (I ended up taking one yesterday afternoon after that awful run), but it can be hard to tell! Sometimes, when I'm totally not feeling it, I end up having a great workout, and it turns out that was just what my body needed -- a little kick in the pants. And some days, I truly need more rest.
Even elite athletes have bad days. I read an article once about how many elite marathoners drop out of races because if they're off-pace, it's not worth the exhaustion to finish. These people are paid just to show up and run -- and even with top-notch training and nutrition, they have slow days too!