I've started reading Running Until You're 100 by Jeff Galloway that Santa brought me for Christmas. Here is a statement from the book that I like.
Mature runners who have time goals can get too focused on running faster almost every run. This often results in injury. One of the first signs is increased stress associated with running, often resulting in motivation problems. At the first sign of these symptoms, reduce mileage and let mind and body get back together again:
Running is not as enjoyable.
You don't look forward to your runs.
When you say something to others about your running, the statements are often negative.
The negativity can permeate other areas of your life.
You look on running as work instead of play.