Easy days suppose to be easy and hard days hard

During my 4 years of running experience with being self coached and trained by coaches it just recently has occurred to me what it really means "easy days easy, hard days hard".

So what does it mean "easy days easy"? Easy days are the days when you run in conversational pace (if you could easily talk in full sentences and your breath is not interrupted) or slower.

Jack Daniels talks about benefits of easy running in his book Daniel's Running Formula (third edition):

"...the first is that you build up certain degree of resistance to injury by taking it easy in many of your runs." "Easy running does a good job of developing the heart muscle, since the maximum force of each stroke of the heart is reached when the heart rate is at about 60 percent of maximum." "...easy running is a good developer of the heart muscle, and although it doesn't feel as if you are working very hard, your heart is." "Another benefit of easy running is an increase in vascularization (opening of more tiny blood vessels that feed the exercising muscles) and development of characteristics of muscles themselves that are involved in running."

The other reason to keep "easy days easy" to keep your body well rested before hard workout.

I workout 6-7 days a week and easy days are very important for me. So I usually run 7:50 to 9:30 min/mile on my easy days, depending on how I feel, what weather conditions are and what workout I had the day before or the next day. And going super slow on my easy actually helped me run my marathon with huge PR (3:04:35 ~7:00 min/mile).

Hard days, on the other hand, suppose to be hard also for a reason. You want to break down your body to teach it recover faster. In my last training season (for Chicago marathon) I practiced having 2-3 workouts on my hard days. For example, morning track workout, afternoon strength workout with weights and later biking or water jogging recovery session.

Balance training is very important in running improvement and injuries prevention.