In reply to Dom Mackenzie:
This is purely off the top of my head, I've not done this or looked at a map so it may well not go.
How about, Fort William to CIC hut, up onto the Aonach's all the way along to ridge to the Grey Corries and stop in the Stob Ban Bothy.
From there across to the YHA Corrour (is it still open?) I think this section might be quite flat.
From Corrour to Ben Alder Cottage via Carn Dearg and Mam Ban (I have done this bit and it quite good).
Ben Alder cottage over Ben Alder and out to Dalwhinnie. Ben Alder is very good but the skin/walk out is a drag.
You could stay at the Raeburn hut just outside Dalwhinnie or end your tour here. I'm not sure if there is any bothies within a days tour of Dalwhinnie that could take you on towards Aviemore, if there is it would make quite a classic tour.