Last Friday my wife and I rode from Ullapool to Bonar Bridge – 60km. This is the shortest coast to coast crossing and is about 42.5km off-road, 17.5km tar. Took 6 1/2 hours including lunch and snack breaks, 5 hours riding.
I've sent route details to OffroadAdventures-Online so it should appear there. Photos at Anny's Bike Ride. They're not in order unfortunately and I've just noticed some from the Dava Way are included.
Not technically difficult, double track most of the way, not too much climbing, some fun downhill. Weather was good, we only got rain in the last mile.