This last week seemed to be an ongoing object lesson in the holism of training, from Tuesday evening at the track (and the immediate thereafter) on to the Monday morning after the Bushwacker 5K. So maybe it's high time to talk a little about what can (help) cause bad days to happen.
Sometimes they do to you. Often times they do to me. But this is about you.
Your bad day can (or may, or may not...) be the result of a number of short-term and longer-term decisions. Everything from the number of adult beverages you drank in the past few days, to the food choices you made in the past 48 hours, to the effort level of the activities you did (or did not) in that same period of time can either positively or negatively reinforce your energy level.
So, for example, if all of your workouts consist of the same activity, for the same duration, at the same intensity level, it's highly likely you have trained yourself not so much into a groove as into a rut. Keep going long enough and it's more likely going to feel like a grave with two ends missing; 'cause you're going to feel like your running is flat out dead. A friend of mine had a bad day on Saturday (in his own opinion); rather than write it off as a bad day, or take it as a sign he might want to adjust his training, he walked around at the post-race party in a severely despondent state, one step (barely) above suicidal.
Outside stressors can cause your run training to be a little less than fun, too. Look for a job, or try to sell a house, or deal with a familial issue for an extended period of time...that stuff will weigh on your mind and eventually affect your physical performance. Sure, at first the run or bike or swim is a brief escape from the stress, but what I'm talking about are those seriously extended circumstances...the ones that can last for six months or more.
So, I have no magical silver bullet for you today. Sometimes, though, knowing where the problem might lie can be the first step in fixing (better yet, understanding) the bad day problem.