Believe it or not, the vast majority of your marathon training mileage should be “easy” running. Easy mileage is everything that is not a tempo, interval, fartlek or long run.
Literally, those miles should feel easy. . . . No matter at what speed the pace charts say you should run them.
Ignore the pace charts.
Leave your watch at home if you have to.
Just run at a pace that feels easy. Enjoy yourself.
Even just a little too much effort on “easy” days will do more harm than good. Conversely, easy mileage at a truly easy pace serves a number of functions and will help get you to your goal marathon pace come race day:
1. Physical recovery – Not every run can or should be a tempo, speed session or long run. The body needs to recover from those efforts before it can successfully repeat or build on them.
You may ask then why we simply don’t rest on the days between our tempo, interval and long run days. That is a very good question, and in fact, some programs eliminate what they call “junk miles,” thereby offering the runner a 3-day-a-week marathon training program. The problem is that easy running serves many functions besides physical recovery. My own experience (and the experience of others I know who have tried 3-day methods), is that without the “junk miles,” I, at some point, end up horribly injured.
2. Muscle Flexibility – One of the problems with simply resting on “easy days” is that the muscles which you worked so hard on your “hard days” grow cold and stiff when you simply sit around “recovering.” Conversely, easy running allows the body to send blood to those damaged muscles, stretching them, providing for a more efficient recovery and eventually, better running form.
3. Injury Prevention – Hand in hand with muscle flexibility, easy running contributes to healthy running. Renowned Author Jack Daniels PhD put it this way, “. . . you build up a certain degree of resistance to injury by taking it easy in many of your runs.” 
4. Mental Recovery – It is no secret that serious marathon training can tax both the body and spirit. It requires a great deal of discipline to keep at it over a period of months, sometimes losing sleep, saying no to social engagements, etc. If we are not careful, we can easily become mentally over-trained. We find ourselves resenting our running schedule, taking our frustrations out on those around us. We begin to hate the sport we once loved.
Easy running helps guard against this mental burnout. On easy days, it literally does not matter how slowly we run. We can take our time, enjoy the scenery and come back hopefully refreshed for our next serious effort.
5. Endurance – Easy miles absolutely count towards our total weekly mileage, which more than any other factor (even the length of our long runs) helps increase our overall endurance.
6. Improved Running Economy – Running economy is defined by Hanson’s Marathon Method as, “The amount of oxygen a runner utilizes to run a certain pace. The less oxygen, the better.”  Daniel’s Running Formula sheds further light on how easy running causes this result by describing how easy running both builds the heart and allows for more oxygen uptake and utilization within the muscles themselves. 
During my training leading up to my Boston Qualifying race, I ran my easy days at a pace sometimes far slower than the various pace charts recommended. At times, I was tempted to fight against a pace that felt easy. The number on my watch looked so slow. I would struggle with thoughts like: “Shouldn’t you just be walking at this point, you are going so slowly? Or, I hope nobody I know sees me running at this pace. This is embarrassing!” But I continued to run my easy days based on how I felt. I believe this strategy left me fresh for my next tempo, interval or long run. I am glad I didn’t let a number on my watch dictate my training plans. If I had, I may not have reached my ultimate goal of Boston Qualifying.
 Jack Daniels, Daniel’s Running Formula, 3rd Ed. (Champaign: Human Kinetics, 2014), 47.
 Luke Humphrey, Keith Hanson, and Kevin Hanson, Hansons marathon method : a renegade path to your fastest marathon, Kindle Edition (Boulder, Colorado: Velo Press, 2012), location 664.
 Daniels, Daniel’s Running Formula, 48.