You may be having trouble pinpointing your AeT, but otherwise I think you have been doing things mostly ‘right’–so don’t give up hope here. As you may know, as an ultra distance runner the most important type of training is relatively high volume, low intensity runs–consistently over months/years. This appears to be what you have been doing. Granted, adding a judicious amount of hills, ME, and 10K race pace (AnT) work is also important at some point, especially about 1-2 months out from a race, but since one’s body often adapts to this type of higher intensity work quite quickly, I think it has been ok for it to have waited. (Others recommend some earlier application of things like hill sprints and ME work, which I respect, but I won’t digress about this).
I would suggest the following:
1. perform a few AeT drift tests indoors on a treadmill keeping the pace constant after warming up and achieving a presumed AeT HR (try 140). In contrast, if you want to perform an AnT test, perform this outside on flat terrain, preferably even racing a flat, local 10K and use this as your AnT test. Maybe post with your results of the treadmill AeT test at 140?
2. Since you are about 2 months out from a possibly hilly, trail ultra, and have only been performing slow, presumably mostly flat runs, I would regardless of your AeT/AnT percent spread, just start a cycle of hill, ME, and/or AnT work-outs–say about 10-20% of your training time. I wouldn’t let your trouble determining your AeT stop you from racing in a race you may become ready for, since you have been doing so much Z1/2 base miles.
3. Don’t quote me on this, but I think your training over the past months may have been at too easy a pace. Most of your time should be at what should feel more moderate than easy, IMO. This is often, if not always, below your AeT. Keep in mind the feeling of moderate should be what your heart (ie HR) and lungs are doing and not your legs per se. Folks often go too easy because they feel like their legs can’t go all day at a certain speed, while it’s more the heart and lungs we are looking at in regards to AeT. (granted beginners, which you are not, are often more limited by their legs, even at short distances). I’m guessing here but I bet your AeT, which can vary a bit based on internal and external conditions, is about 10-15 points above your MAF at this point–perhaps in the 135-145 range.
As you may know, in the end, your body adapts to what you do with it. If you have only been running super slow over long, flat distances this is what you are good at right now. Other speeds, terrain, and sports, you are probably de-conditioned. But as I mentioned above, building up an aerobic machine to move all day long at a slow pace takes much more time to build, as well as more time to de-condition, while other things like speed and hills can come and go relatively more quickly. My point here is you have already done the more time-consuming work and are likely in a good jumping forward spot to polish it off.
Best of luck. Hope some of my recs are helpful.