In reply to Rimz:
Hello, Rimz--I've been as far as the corrie below Maoile Lunndaidh in Gleann Fhiodhaig from Achnashellach. As far as that point, it's certainly cyclable on a mountain bike.
I remember this well, because I was cross I hadn't brought mine. I never walk a track if I could be cycling it--the faster you're going, the more you can do with the day, and the less you wear out your moving parts, too.
The track gets pretty minimal at the watershed, but it's mossy grass, so it would be easily doable in dry conditions when there's no snow. In fact, one of these days I plan to do it myself.
What happens after that point I don't know, of course, but since by then you're already going downhill towards Scardroy, and since I've seen landrovers and the like in the area, I would expect the track to improve, and I'd be prepared to gamble my day's plan on this expectation.
Post edited at 19:09