I used to make the classic mistake of running my easy days way to fast. I would run like 6:45-7:00 for my easy days. My HR would be like 180+ (about mid Z3). Now at the start of this spring/summer base training I decided to try training off of mostly HR. At the start I'd be running like 8:30/mi at the top of my Z2. Now I can run like 7:45/mi and have a 10 bpm lower HR. Basically I improved my top of Z2 from 8:30 to like 7:30. This worked for me. It takes a lot of work to TRY to run slow on easy days. I now run my easy days around 7:45-8:00 most of the time.