Great to hear of your improvements and good work sticking to the training. The benefits in the mountains are tangible as you’ve seen.
You can use 156 as the upper limit for Z2 aerobic work now. The only thing you may find is that if running on the flats you might experience significantly more muscle fatigue at what will surely be a faster pace. In that case you may be wise to restrict the AeT running volume to 1-2 hours/week and do the rest at AeT minus 10-15bpm. If you are hiking on hills then you probably will not find 156 as fatiguing as when running. Just be aware that you will be doing a bunch more work at this higher HR so will want to factor in more recovery workouts.
As for ME. If you are using the ME as explained in the article and video on this site then load yourself up enough that the legs are the limiter and HR will probably be relatively low compared to how hard you feel you are working. Be sure during the ME period to keep the aerobic volume high. So lots of Z1 and Z2 workouts.