It's not that you don't ever run hard...because their is definitely a time and place for that. It's just that slowing down often allows for aerobic development that harder running can actually hinder. Often...slowing down...relative to a habitual training pace...is temporary as your aerobic system "reboots" and comes back into balance. Once balanced you come back to your previous training paces...but with much less effort.
Paradoxically...the only runners not training too fast are very often those that are actually running the fastest...relative to others. Yes...you read that right. The best runners are usually very much within themselves. Even though they are moving "fast" in comparison to others...their effort is easy relative to their capacity. But all the people chasing tend to overreach.
Very often the person working the hardest...is the one who is the "slower" relative to others. I observed a group run a few weekends ago. At the front of the group...the runners looked strong and fresh and well within themselves. At the back...after having fallen behind the group...were a few runners who were killing themselves trying to keep up. They were virtually racing.
This article does a great job of explaining why running easy makes you faster...and what your easy pace should be.
So if you find yourself stuck..or just not running well in general...the first thing to try is to slow down your easy days. See what happens. And if slowing down your easy days means that you are walking...do just that...add walk breaks. Experiment.
In "Way of the Peaceful Warrior," by Dan Millman...the character Socrates taught Dan that the three constants in life are Paradox, Humor and Change. I guess slowing down to go faster falls under the category of paradox