Both pace and HR at an individual’s AeT are the best ways to measure basic aerobic fitness out side of a lab. This quality is extremely “trainable” as opposed to maxVO2 which is what’s called a “first wave response”. maxVO2 responds to training very rapidly in the young or untrained individuals but response to training diminished with training history. Athletes with a long training history will most likely have reached their genetic limit for maximal oxygen uptake after a few years of training. Luckily, maxVO2 is only one of several physiological components that determine one’s endurance performance and does not have as strong a correlation with endurance as do Lactate (Anaerobic Threshold) or Economy, both of which are much more trainable.
A small digression:
The aerobic base, as measured by the AeT provides a critical base of support for endurance especially as determined by ones Lactate/Anaerobic Threshold. Improve AnT when you have a low AeT and you are overlooking some easy endurance gains. Raise AeT and you will be able to raise AnT even higher.
Back to your question:
Find a place you can return to frequently and conduct your own personal AeT time trial/test there every few weeks. Maybe this a local trail or long hill you can run/hike up at your AeT HR for 30-45min or even longer. Hold the HR to this AeT limit and time yourself. Come back in a few weeks when you are in the same state of rest/fatigue and repeat this test. You’ll see the time improve over a long time scale. Since this is a low intensity test you can do it as part of a regular running circuit and do not need to treat like a more taxing high intensity time trial. Over the course of weeks and months you will see the time drop for the course. You should also notice a decrease in perceived effort (lower respiration rate). Voila: Your basic aerobic capacity is improving. At this same time the AeT HR will move up. You can use a ventilatory test like nose breathing or conversational pace to see that the AeT HR is not a bit higher. To really nail down the new AeT HR takes a lab test or a field blood lactate test.
To help inform your training, do an AnT test as described here: https://uphillathlete.com/diy-anaerobic-test/ and see what the spread is between AeT and AnT. Greater than 10% in terms of HR? Then you still need more aerobic base work. Less than 10% then you’d benefit from adding more high intensity.