There’s a crafty bit of rationalisation in the Castleden book , initially he says “there was a lowland route connecting Skara Brae with the ceremonial centre on the isthmus “ then in the same paragraph the route becomes “, the road turned east along the southern shore of the Loch of Harray …..” . We can all find routes but a bit more difficult to find an actual road .
He also mentions the Sarsen Road from Avebury to Stonehenge ( in the same chapter ) equally as subjective and with no archaeological evidence to support it at the time of writing (1987).

The old Sandwick Road ran by the western edge of the Ring of Brodgar and up the Black Hill of Warbuster between Skae Frue and the western side of the Ring of Bookan henge. And the other way is the supposed causeway between the two stone circles. Don't know of any track between these and Maes Howe though. Anything else is historical speculation apart from stone quarrying routes it seems.

A slightly different site, but I recall reading about a low raised earth bank described as a 'processional way' between Gibb Hill and Arbor Low.
Dunno if this would qualify as a low road, but I've read of similar 'tracks' between other sites. I'm sure there were many, now lost, routes of interconnectedness between related ritual and domestic sites.

You can see the earthwork between Arbor Low and Gibb Hill quite clearly on google earth/maps.