I've walked the lot, a few years ago now. People do cycle it. The section at the north end of Loch Lomond is carrying territory - up and down lots of little steps & boulders.
Boyfriend has cycled from Tyndrum to Fort William over two days a couple of times. I think that section is all fine (apart from a couple of climbs, but that's not really what you're asking). (B/F has also run the whole thing in one go, but that's just craziness...)