Description
It was only after cycling this route and noticing how smooth it was that I looked at some old maps and discovered that this was indeed a disused public road!
https://www.sabre-roads.org.uk/wiki/index.php?title=A87/History
https://maps.nls.uk/geo/explore/#zoom=13.0&lat=57.13467&lon=-5.13090&layers=202&b=4&o=100