I think it works this way:
If you donīt have much time: go as hard as you can when you can.
Plenty time: Altenate going hard with days where you use your measured MaxHR and stay below 75%.
Going hard as you can preferably with some intervals will make you fitter and leaner than staying in some zone, the idea of zone training is to prevent overtraining - that is if you have the time for overtraining.