Gaining and losing fitness?

So I get that in order to boost your aerobic engine you need to do a mix of intensities eg. zone2 and high intensity in the polarised model.

When you come off a training program and just cycle, do you (generally) maintain all of your fitness eg. fat oxidation capacity, VO2 Max etc if you cycle at threshold and above or do you lost certain aspects of fitness. I.e. Do you need to train your zone2, threshold and VO2max to keep your capacity in those zones?

You pretty much need to hit all the zones in order to maintain all the zones, I think, but you don’t need much time in zone in each zone to do so.

Hope you didn’t zone out during that.