Like Oblongbob, I did it N->S, but B&Bed it over 3 days, which was pretty civilised. 2 days would be doable if you were happy with early starts and late finishes and you could travel light.
Getting food en-route was less of a problem than expected, and I brought a lot of the snack stuff I’d taken back home with me – although obviously I was bookending each day nicely ballasted with a full cooked breakfast and dinner in the pub.
I did:-
Inverness to Laggan, stopping at Fort Augustus for lunch. Stayed at the Laggan Hotel. This was the most interesting and technical day’s riding. Correyairack was a bit of **** to ride up from the north and a tough descent on a gravel bike. You’ll definitely be pushing up it from the south, especially if you’re loaded. Nicer descent though.
Laggan to Killin stopping at Courrour Station (didn’t know about the Glen Lyon Tearoom). B&B in Killin. Probably the best scenery of the trip – felt proper highlandy, sweeping glens and mountains. Not technically challenging though
Killin to Glasgow was pretty easy, bar some of the WHW near Glasgow being very battered. Lunch in Aberfoyle and Loch Venachar to Aberfoyle was good riding.
As above, the wardrobe on the train can only take 2 bikes (at best – not designed for modern bikes at all – my Fugio barely fitted), so make sure you have a place booked for the train you’re on.