It really is all about what an individual is able to handle physically, and that's why I hate the boxes people put on themselves about easy days. My running career started when I was trying to play both baseball and run track. Every day after baseball practice, I would come home and run 4-5 miles as fast as I could. This was around my junior year of high school, so I was running them between 5:45 pace and 6:15.
The approach just kind of stuck...from 45 miles, to 55 miles, to 75 miles, I always gravitated back to roughly 6:00 pace. Everyone on my college team argued with me about it, and I did plenty of runs by myself because no one wanted to run with me. In the end, I went from being a 9:50 2-miler in HS down to running 8:30 for the 10k and eventually running a 2:22 marathon in awful conditions. Even when I was doing my heaviest marathon training, plenty of my daily runs were below 6:00 pace, and never had a stress fracture or major injury. It was painfully boring to run 6:30 pace on my easy days.
In the end, I was always able to handle high volume at a high intensity. Sure, there were some days where I felt absolutely worn down, and those were the days I ran slower. I just always ran as fast as comfortably possible on most of my training runs and ended up having a very good career. Bottom line is that you have to listen to your own body instead of the constraints of other people whose bodies might respond differently to training stimuli than your own.