It's been a revelation for me. Run the easy runs easy and the hard runs hard! So simple a concept, I'm not sure why I never grasped it earlier. I use the 80/20 running calculator and go by pace, not heart rate.
I used a coach this spring and it was a huge revelation for me as to how unnecessarily hard I had been running. Wow.
I'm running more now than I was when I joined this place, back in the naughty aughties. Thanks, Zone 2.