Here is my story, maybe it will help a little.
When I was in college I was pretty good, D1, a few AA awards, sub 8 indoor 3k and well sub 14 outdoor 5k. I heard from several people who had been to or coached athletes to this level that I had the tools to run 3:38-39 and 13:15-20 or so. Generally speaking I ran 7:00-7:30 pace on all my in season easy runs and long runs. This was two workouts, 3ish doubles a week, and a long run, pretty standard.
When I finished college I thought maybe I had what it took to go to the next level. I joined a group of like minded athletes, and trained harder than I ever had. Three workouts a week, 4ish double days sometimes two hard workouts a day, and I was running all my easy mileage at 6:30 or so pace, this includes my long run, 15, 16, 17+ mile runs at 6:30 or faster pace. What happened you ask, big prs, serious runs at national titles, world cross teams, road race glory?
I got slower, a lot slower. I would consistently hit 3200 in a 5k or 5k in a 10k at pr pace feeling like I was jogging, and then immediately fall apart. It was extremely frustrating, and as a compulsive runner I just kept training harder, and racing slower. This problem was compounded by that fact that I was surrounded by other athletes who did this too, with varying levels of success, and a coach who encouraged training as hard as you can always. Ultimately it's my fault as I should have known better, but here's the lesson, harder is not always better.
This applies not only to easy days, but to workouts. To my mind, and when I was doing it, 6:30 pace felt no different than 7:30 pace. Also, there's not a huge difference between say 5:00 pace tempos and 5:10 pace when you're running it, but your body knows better, and it will come back to you later. When you're doing a great interval session, stop with one repeat left in you. Here's why, your body only has so many all out efforts in it per year, if you're using them in workouts, you'll have great workouts but come up short when the real racing starts, like I did.
Take what you want from this, it's just one story.