Buddy, your coach is right. You get the same benefits running at 7-7:30 min mile pace than 6:30s. Running faster on “easy” days does not make you faster or improve endurance any better. The reason why he wants you to run slower is so that you can sustain it longer. The key about easy/normal mileage is run as long as possible on a consistent basis, not faster. Running slower makes it easier for your body to recover from longer runs and helps you stay away from injury. Look up Jack Daniels easy mileage lecture on youtube, if you are not convinced.