“how low (obviously “recovery” occasionally is fine) below aerobic threshold is not helpful “enough” for training?”
I think this is a function of duration and not just intensity (that is my understanding from TFNA, anyway). One walk of 8-hours in Zone 1 is very different from doing 8x 1-hour walks in Zone 1. Also, IIRC, some of this is muscular/skeletal adaptation and not just aerobic conditioning, which is also important for avoiding injury when in the hills. For that you also need duration.
Hope that is of some help. Hopefully someone more knowledgeable can chime in.