I have been running for a few months (34m), and I can feel I have become more fit when it comes to how I feel and achieve while running. Now my session is more than double the time than when I started, and even when I end it, I would still be in zone 2 as my heart rate and breath rate are lower, unlike when I began running and I would always find myself at zone 4 by half the time (similar pace). Even when I try to run faster, I can do so for longer than before.
Yet, my VO2 max has dropped from 39.8 to 37.8! I would get very surprised every time I see it drop. Why would that be?
The Apple Watch likes you to kill yourself every run which isn’t the proper way to train. If you do a lot of zone2 running it will tank your VO2Max. I pay it no attention other than for giggles.