Oh how cute, estimates. 220-age is so accurate lol. Just a joke to even talk about this nonsense, it's completely unhelpful in all manner. I just set myself an arbitrary goal for the run (say 160bpm over an hour, which is something i'm working towards so i may be say averaging 165 currently) and try to attain that progresively each week. No idea what zone that is, don't care. If it's currently 'zone 4' so what? It will eventually become 'zone 2'. If i spend all my time doing 'zone 2' at worst i'll be wasting time, at best, i'll be spinning my wheels.