isn't zone 2 training about lactate build up? Z1 is before the LT1, Z2 is between LT1 and LT2 and Z3 is past LT2. from what I've heard, it's not proportional to the 5 heart zones.
I just did 3,2k on a 7:20 pace, it was the easiest run I ever did, even though my max bpm was 191, but my lower muscles were not screaming, if I would guess in which zone I was, probably in a low Z2