The next time you run, try the following experiment. At some random point in the middle of the run, slow down a little—maybe 10 percent. Then mentally compare the feeling of this slower pace to that of your previous, faster pace. You will probably realize that there was a slight edge to the feeling of running at your normal pace that you weren't even aware of until you slowed down. You were straining just a bit without being conscious of it until you took the edge off and gave yourself a basis for comparison.
The reason I'm so confident about how this experiment will turn out for you is that most runners strain a little when running at their natural pace. Studies have shown that when men and women are asked to run at a self-selected pace for a certain distance or duration, they typically choose a pace that corresponds to a score of 12.5 on the Borg Scale of Perceived Exertion, which goes from 6 to 20. A rating of 12 on that scale corresponds to the description "fairly light" while a rating of 13 corresponds to "somewhat hard." So an effort of 12.5 is indeed one that has a slight edge to it—an edge you might not notice until and unless you slow down.
Why do most runners naturally run at this effort level? Sports scientists have generally assumed that runners unconsciously lock into a specific physiological intensity—for example the body's maximum rate of fat burning or the most energetically efficient pace. But research has revealed that there is no consistent link between natural running pace and a specific physiological intensity across individuals. While it's true that a runner's natural running pace is likely to be his most efficient pace, the causal arrow actually goes in the other direction: Runners tend to become most efficient at the pace they run at most often.
The only thing that's consistent with natural running pace is the effort level of 12.5 on Borg's 6 to 20 scale. So it's obvious that runners choose their habitual jogging pace not by physiology but by feel. My theory is that the feeling of slight strain that most runners hone in on represents a compromise between the desire to get the run over with quickly and the desire not to suffer as one would if one completed every run as quickly as possible.
Let's now think about what would happen if you ran 10 percent slower than normal on all of your easy runs. At first you would find it strangely difficult. You are so accustomed to settling automatically into that 12.5 effort that you would continually slip back into it when your thoughts wandered. You'd have to keep catching yourself and forcing yourself to downshift.
More: 3 Rules for Easy Runs
Despite this internal struggle, you would generally feel better when you were running slower. And why wouldn't you? You wouldn't be working as hard. Before too long you might even develop a taste for that slower pace. You would no longer be in such a mental hurry to get the run over with, instead finding yourself content to let the experience last. You might even find yourself looking forward to runs more than you have in a while.
The next thing you would notice was that you felt fresher and stronger on most of your runs, so that you could run faster if you wanted to. Of course, every good running program includes some high-intensity workouts. With most of your runs now being done without even a hint of strain, you would start to crush those designated faster runs and thus get more benefit from them.