Train to Mallaig, track along the north shore of Loch Morar, ferry to Inverie, over and down to Barrisdale, track to Kinloch Hourn (how rideable is this? I've not been into Knoydart this way), over to Glen Shiel to the Cluanie, north, then east towards Glen Affric. Not sure exactly about the next bit, but eventually heading over towards Grantown where you ditch the bikes and pick up canoes, then spend a pleasant couple of days canoeing down the Spey to the Moray Firth, stopping at some distilleries.
I admit that I've only ever done bits of this route, and in other places I'm guessing at how rideable some of the tracks are, and I still haven't worked out the best way after Glen Affric, and the canoe bit at the end is a bit of a twist that would take some planning, but it's something I'd love to do. I just need to persuade my normal riding mates, but they are being a bit sceptical at the moment.