South to North. It would hardly seem like an achievement to “arrive” in Glasgow.
Anything is doable on a rigid geared bike. Front suspension might be appreciated for comfort and for bits like the Devils Staircase descent.
Some folk are doing it one (long) day, so 2 seems like it would be enough.
Google for the West Highland Way
Luggage?
The most difficult riding will be the section up Lomondside after Rowardennan as this will definitely involve some carrying. The rest is mostly on good, made path.
You might want to skip some small sections across fields and the like and just use the road (e.g. from Kinghouse to the DS).