Good questions! Endurance performance is not determined by any one component but by a complex interlinking of several, each of which is the result of an incredibly complex series of biochemical interactions. When you here about things like maxVO2 or lactate threshold you need to understand that these are man made constructs. They are very much simplified models of this complex organism’s response to exercise. Science creates simple models of complex systems to help us understand. As mathematician George Box famously said: “All models are wrong. Some are useful.”
Science tells us that the 3 factors that contribute most to endurance performance are, in order of importance:
1) Speed at lactate threshold. This it maximum sustainable speed relative to the event. This is known to correlate very closely to endurance performance. It is also highly trainable.
2) Economy. This is the energy cost of sustaining race pace speeds. It has two components: mechanical efficiency and metabolic efficiency. Lower economy means that it takes more energy to run, ski or climb at a given speed. This too correlates very highly with endurance performance. It is trainable but takes considerable training focus to do so.
3) maxVO2. This is measure of the maximum amount of oxygen your body can take up and utilize. It serves as a proxy for metabolic power. It can be sustained for nor more than a few minutes. It responds well to training in the young and the unfit and as such is termed a “first wave response” to training. Athletes with many years of training do not normally see much change in maxVO2 in response to training but still see performance gains, even when maxVO2 drops. Of these three components it has the least correlation to endurance performance. However since a high maxVO2 enables a higher lactate threshold, athletes with high maxVO2 have more potential to develop better endurance through raising the lactate threshold. Hence the importance placed on maxVO2.
Keep in mind that the above model of endurance, though widely accepted, is still an artificial construct. These 3 components are, in many ways, interdependent and cannot, down at the biochemical level, be so easily separated.
Where does the lowly basic aerobic capacity as measured by aerobic threshold come into play then? And why do we stress it so much? Why do the best endurance athletes in the world train this quality so much? Read this for some background: https://uphillathlete.com/forums/topic/heads-up-on-nyt-do-you-have-what-it-takes-to-be-an-olympian/
The answer is complex but here is the short version: It is the high aerobic threshold (a manifestation of the aerobic capacity of the slow twitch fibers) that enables high intensity to be sustained for longer durations. The ST fibers take up and utilize as fuel, the lactate that is produced by high intensity work. The ST fibers a like a biochemical vacuum cleaner sucking up all the bad waste products of the FT fibers. The bigger that vacuum cleaner the more high intensity you can sustain, for longer. The more high intensity work can sustain the greater the benefits to lactate threshold and maxVO2 and economy.
Can you see how all these things are interlinked?