I'd recommend staying further west: Ayrshire (Rabbie Burns Country! - gentle terrain, but varied) up to Gourock for the Dunoon ferry. Avoids the A82 which has some cyclepath alternatives in places, but can be a bit grim. Loch Lomond is nice though, I grant you, but up to Inverary or due west to the Portavadie-Tarbert ferry, then to Oban via Kilmartin (stuffed with ancient pre-historic monuments).
There's a pleasant NCN route up through N. Ayrshire and Renfrewshire to the Clyde, on lanes and old railway lines: routes 7 and 75. I've tried various routes through the urban fringes of Glasgow, but they're fiddly and have the usual urban issues of broken glass, missing signs etc. A bit biased - we lived in a rural-ish bit of Ayrshire for a while (Mrs Tomsk's roots are in Paisley), which is a world away from Glasgae.