Mostly covered above I think.
There are a couple of short “off-road” sections such as the Crinan Canal, around Duror and the wee bit into Laggan Locks. However, there is a longer section north of Gairlochy on rough forest track. I managed it on 28mm Marathons, taking a bit of care. Not sure I’d like to try it on anything skinnier.
There is a new cycle/walk path uphill from Fort William, roughly parallel to the road. Don’t be tempted by it unless you are very uncomfortable sharing a road with cars. The path is loose, climbs even higher than the road and has a couple over very steep sections.
Don’t underestimate the hilliness of the first day or so.
Make sure you have the number for the Camusnagaul ferryman. He is normally very accommodating if your schedule changes – unless there is a shinty match on.
It’s a lovely route. I need to return and do it a bit slower next time 😉
https://www.blog.scotroutes.com/2016/09/ncn78-caledonia-way-in-day.html