I've said all of this in the past, and was recently informed by Vern that I must have a strange idea of what is busy. Maybe I do, but I live next to the road, I've seen what it can be like.....Whilst he lives in Leeds!
There's a cycle path next to the bottom half of Loch Lomond, part of the old road. Not sure how good it is, only seen it not ridden it.
There is the track from Glasgow as far as Tarbert, and I'd recommend cycling from Glasgow, except that between Glasgow and Bowling, depending on your route the surface is either great Tarmac or, irritatingly, canal towpath type surface with tiny stones. Facing East for a moment at Clydebank (the main 'junction' between Glasgow and Bowling), the paths split/converge, with the path along the canal and to the East being tiny stones, and the path to the south and through Glasgow 'proper' tarmac.
Facing west again however, you continue along the side of the canal (The Forth and Clyde Canal) to Bowling. I take the lower road through Dalmuir and Old Kilpatrick (where the Erskine Bridge is) as the path is exclusively the small stones surface to Bowling. This is an absolute B*GGER, because if it was surfaced properly, that would be a fantastic section. As is is, its my least favourite section by a country mile.
That said, you don't need to go on the A82 at any point, in fact, you don't need to at all until north of Tarbert, it really is as simple as.
From Bowling to Tarbert however, it is all tarmac, and, save for a couple of short detours in Dumbarton, Balloch and Luss, its ok by and large (I cycle with puncture prone skinny tyres on it all the time AND I AM MORE LIKELY TO GET PUNCTURES ON THE ROAD, seriously!).
It does have its little…. characteristics though, such as a very short bit between Dumbarton and Renton where you can say hello to some cows, depending on the time of year.... the single bit of pavement at Alexandria (Bonhill Bridge) which seems to have been placed across 3 quarters of the path at a right angle for no other reason than to catch daydreaming cyclists out (Although its nothing if you aren't speeding along at a ridiculous rate of knots with skinny tyres like I was the first time I hit it and squashed my front tyre!
), the short bit, i.e. about 50 meters at Balloch where some twat has decided to lay gravel and.... well, the bit between Balloch and Luss is ok, but actually it is the bits of the pavement beside the A82 that seem to need clearing (possibly crap off the road?) the sections of old road and footpath are ok though, there are even one or two short stretches of scenic road thrown in for good measure such as through Arden (I think it is).
Balloch to Luss seems to be a slightly odd mishmash of things, and its clear they had to rustle up a path from what they had!
After Luss, going to Tarbert However, I actually found this to be the BEST BIT of all (except for Bowling to Dumbarton). Largely on the old road, I really like it, and you can stop and look at the view to your heart's content, all without getting mown down by a passing lorry, which, after all is the whole point of Bowling to Tarbert isn't it? You hardly use a road the whole way!
There, what better info do you want?
The bit next to the Loch past Tarbert used to be an awful road surface, probably better now but be prepared, it's also narrow and you get some big lorries and coaches down it, so listen out for them approaching.
The A82 is the main route north, in that part of the world, and due to the geography north of Tarbert, they can't widen it, so its this horrible, winding, twisty road with just about everyone and their dog using it.
That is one bit I don't think you can avoid however.
