I had absolutely NOTHING today. Legs were dead; no pop and no drive, and faster than normal heart beat. The messages were clear as that feeling continued even after warming up so I made sure to do a short run (40 min.) and headed back home. I call this the "red zone" and this is where runners have 2 choices: listen to your body or push beyond and significantly increase the chances for injury; I've learned to choose the former :). There's always tomorrow and nothing beats injury free running.