I already use my HR when I’m on the trail, as you suggest, but have problems to keep the given Zone for a training pass, typically on steep climbs. E.g. if I have a Running Foundation pass my HR easy goes into Zone X and Zone 3, even if I slow down, to a fast walk. Of cause I could slow down to a very slow walk or stop until my HR is in the requested zone, but that does not make lot of sense to me. Some of the terrain I’m running in has climbs on more than 20%. If you would be out on a RF pass, how would you handle next Km if you know it has 15-20% gradient?