When I was young, I recon, I did every run-in zone 4. Without really knowing. It took a while, but I can now do all easy recovery runs in zone 2 below 130 beats for me. The pace varies between 6;30 and 8;30 minutes per kilometre depending on the terrain and conditions. My top end speed and parkrun time have slowed suffered. Hopefully I can add some more speed back over time. I get the feeling it is a slow process the older we get.