alternative method to determine aerobic pathway focus?

If the drift test hinges on pathways being different before/after the ~.5hr. mark, does that mean that any run significantly longer than that will necessarily be training the aerobic pathway, and one doesn’t need to worry so much about the numbers?  For instance, if I go out for a 2 hr run, can I feel comfortable I’m training my aerobic system, irrespective of my HR?

Background:

49 y.o. male with history in most mountain/adventure activities, never at a high level (but climbed the Nose, technical route on Denali, kayaked the Grand Canyon, etc., so not completely “suburban”), and with genetics(/willpower) that at my peak in high school never brought me below a ~18′ 5k, and currently leaves me stuck with a ~44 VO2Max.  Always a recreational runner, I’ve been more focused over the past decade (kids pushed out adventure), running ~20 miles/wk over 3-4 runs, less consistently than I’d like since… family (e.g., we all seem to get sick six times a year), but still probably fairly consistent by most metrics.

I’ve read TftUA, listen to the podcast, and after playing around with increasing my Zone2 runs, I made a formal effort last spring/summer.  Though I assiduously stuck to my plan, I never saw an uptick in my AeT, perhaps because I had so emphasized long/slow previously that I was not actually aerobically deficient, at least relative to my general fitness.  FWIW, my AeT was, and still is, ~140bpm, which I achieve at an ~11:30 mile. (I feel like I should be embarrassed to say this, given how hardcore folks on this forum are, but, well, that’s who I am.)  In fact it actually went *down* as I devoted 2.5 months to concerted Ae training, perhaps because I was dropping off any ME and power I may have had previously. (Other theories?)

Much of my question comes from the fact that my HR feels so unreliable: I can some days go out for a 2 hr run around my AeT and watch the HR and pace both drop consistently into my second hour; other times I can, after two rest days, plenty of sleep, and a watch giving me the HRV thumbs-up, and I can go out on a run and watch my HR climb to 160+(+) even while I slacken the pace, then slacken some more (repeat).  Like most of us, I feel both like your drift test and advice on the podcast is all so straightforward, and I appreciate it so much… but then my own experience seems so unrelated to it, and so all over the map.  So I’m wondering if, instead of thinking so much about the numbers (not that I get unhealthily worked up about them, or even geek out about them… but I at least strive to attend to them), I can serve my aerobic capacity almost as well, or perhaps even better, by just saying something like “anytime I run for at least n minutes, I can feel comfortable I’m training my aerobic system.” And n ~= … ?