Every run has a purpose. There is no good or bad run. However, for some, a bad run would be when they fail to meet their expectations for the run.
Here are some things you can learn from a bad run:
Every run is essential. It teaches you more about yourself. Learn to face the bad runs and not fear them. They're all a part of the process.
"The most successful runners are the most confident runners. And the most confident runners are the most flexible runners. Remember that every training plan is written in pencil… with an eraser!"