I think your are smart to do this even if only for the health benefits. A high volume of low intensity aerobic work will contribute to basic aerobic capacity….up to a point. Because the stimulus is low with such a low HR it is not going to have nearly the same training effect as an hour of running at HR=130 say. It is actually going to act as a recovery workout and may speed recovery. As to how to count it: Id call it recovery and keep a tally of the volume of that intensity zone separate from Z1 and 2.