A large majority of our miles are made up of "easy" miles. What "easy" means to an experienced runner versus a newer runner is vastly different. The problem with calling runs "easy" is that this is completely subjective. It is meant to be valuable in that each person can individualize what feels easy for them on different days. Some days my easy may be 8:00 minute pace, some day it may be 10:00 minute pace. It is important when running easy, that ego is completely set aside and the pace is genuinely easy. Subjectivity is a fantastic thing in determining levels of difficulty, but it requires that a person be self-aware and have a secure ego.
Calling for easy days backfires when people:
1. Rigid understanding of what SHOULD feel easy to them, and don't take each day in it's own right.
2. Difficulty relinquishing control of the watch.
3. Black & white thinking: they convince themselves that because something isn't hard, it means it must be easy.
No matter how much science a person hears about polarization, supercompensation, recovery days, and achieving gains by slowing down their easy days, if somebody is not willing to accept this as truth and relinquish whatever psychological need they have to move quickly, they're not going to internalize those lessons.
Running slowly on easy days will not sacrifice any fitness, and will only allow you to develop gains from your hard days.
Running at the far ends of the spectrum is what propels fitness gains (i.e. running really fast on hard days, and taking easy days really slow).
Taking rest days is imperative, but many people have a hard time sitting still, doing nothing, and emphasize the "active" in "active recovery."
People confuse what is not uncomfortable to mean that it is easy. This is not the case. Just because something doesn't burn you, doesn't mean it's not hot.
How can we get people to ACTUALLY go easy on easy days?
Run by heart rate. This definitely works some of the time! However, when somebody is very aerobically fit, or on an easy day that does not follow a hard day, someone's heart rate will probably not rise very much at a too-fast-for-easy pace. So, assigning heart rate runs is very useful for the day after a hard workout, but what about the other easy days?
Run by time. This definitely works initially, as people stop focusing on getting the miles done and start focusing on just running for time. However, after a while...people may start trying to see how many miles they can finish within that set period of time. Or people may keep tabs on how many miles they completed during their previous 40 minute run and try to match or beat that rather than meeting themselves where they're are and running relaxed.
What should easy days really look like? Easy days should actually be called TOO EASY days. Run what feels completely too easy and is almost frustratingly slow. Running a comfortable pace is too fast. This pace should be more comfortable than comfortable.
A recovery run should not be the same pace as your long run. It should be SLOWER than your long run pace. A recovery or easy run should use little to no glycogen from your muscles; it should promote recovery, not prolong it; and it should promote fitness gains, not create any muscle damage like your long runs or speed work will. Also, the length of an easy recovery run is important. If you're running at a high level with high mileage (let's say 75+ miles per week) your recovery runs may be up to 10 miles. However, most of us are not at that level, and running this duration or longer will prohibit recovery rather than promote it.
*One caveat to easy running pace is that some people may perceive that by running slowly they feel flat or their legs feel more tired. This is not most people. And even those people, need to slow down their easy runs and find that magical niche that allows them to promote recovery without feeling flat the next day. It also must be reiterated that easy running pace varies for everyone from day to day, there is no specific pace that should be run every time. That is why there is often a wide range identified.
Bottom line: slow down to run fast. Take the easy days too easy. It will only help you.