Today has been the day when I figured out the route. It will take about 13 days and be something like this:

Montrose - Edzall - Glen Clova - Braemar - Glen Feshie - Kinguissie - Laggan - Fort Augustus - Glen Garry - Glen Dessary - Camusrory - Inverie

On the way there will be wild camping, bothies, and maybe the occasional more luxurious accommodation. At present count the route also passes two pubs.