I’ve been saying “Easy days Easy and Hard days Hard” to athletes for a good ten years.
What that means is that you need to run your easy days easy so that your hard days can be hard. You want to make every workout day a good one and so you can’t be running moderate pace on your recovery days (unless you’re world class and have a high training age, and even then you can only get away with doing this a couple of times a month).
Running your easy days easy so that you’re ready to run solid workouts on the hard days is a very simple concept, but as Thelonius Monk said, “Simple Ain’t Easy.”