The ‘grey zone’… my take.
You are supposed to train easy or cap the session at a set HR but you don’t take note or you let it drift a little higher than it should…
This means you are training harder than you should and your effort and the effect on your body is slightly greater than initially intended…
This process repeats
Then, you are due for a key interval session aiming to create a physiological response, but, your body is carrying fatigue (and maybe your mind) and your efforts fall below what was initially intended
All of a sudden there isn’t much difference between your scheduled ‘easy’, ‘lower intensity’ sessions and your ‘hard’ sessions… the majority of your training (unintentionally) in a middle range… the ‘grey zone’
Solution?
Follow your program, talk to the Coach, ensure you ‘use your gears’ and develop a robust range of intensities.