FATMAP difficulty grade
See Dave Banks website for a description of the route. Mostly farm track/landrover track with some tarmac and a little bit of singletrack.
Nothing too technical or difficult but there is some ascent in the route and some parts can be a bit muddy so best on a mountain bike or cyclocross.
It is around 41km or 25miles in total.
To summarise his excellent report on the route: park near the reindeer centre, go clockwise.
A similar route is described in Harry Henniker's book "101 mountain bike routes in Scotland"