Look into aerobic and anaerobic thresholds.I do not. Zone tends to be 2 when walking purely cause I start it as a run and when it goes to Z5 I stop. I'll need to have a further delve into everything to see whats best to increase the fitness.
Aerobic is where the body uses fat to supply energy and anaerobic is where it starts to use carbohydrates. Carbohydrate fuelling is what leads to lactic acid production. Your heart rate determines which system your in. You start to feel tired/hit the wall when your body is producing more lactic acid than can be processed by the body.
That is my very basic understanding, and there is loads and loads of online resources.
How have you calculated your max HR? 220 takeaway age? Or have you done a test?