As soon as you sign up for a marathon you start to think about what it will take to finish it. You come up with a plan. You design a series of of workouts. And you realize you are going to have to suffer.  

Before you even begin you first workout, you convince yourself that success will require pain and suffering.  

But if you maintain your marathon mindset and you just tell yourself you're supposed to suffer you will suffer yourself right into an over training injury. And then you won't have days of workouts to reorganize. You may wind up with weeks or months of completely avoidable time on the couch.