We (my wife - fiancée then - and I) did some of that route - well, up alongside L. Lomond, then a wide loop taking in Inveraray and Oban and up to Glencoe and Rannoch Moor, back in 1980! Since then the road alongside Loch Lomond has been upgraded, widened and straightened, and no longer winds along hugging the 'bonnie braes and bonnie banks' right alongside the loch, as it used to. I believe some stretches of the old road are still in place and can be cycled upon.
Apart from that, little alternative if you want to stay on roads, I'm afraid. In the Highlands, you have to be either a roadie, used to fast traffic, or a complete off-roader, used to rough stuff. There's no middle ground.