Err that route up the east side of Loch Lomond?? Thats the West Highland Way! Now I've not been up there in over 10 years but it was no where near cycleable when I did it and its a footpath, for walkers not a level path.
This! I've walked it 3 years ago so don't know how I missed it! Agree with Dave it's only suitable for walkers or a real MTB nut (who would carry the bike most of the way anyway, path here is little more than a sheep trail).
I'm not convinced about the Rest and Be Thankful, all the traffic heading west in direction of Inveraray, Campbletown and Lochgilphead (and ferries to the inner Hebrides) is on that road. A lot of lorries, caravans, coaches.
It's a bit of a bind though because the only roads East-West here are the A83 Rest and Be Thankful or A85 Killin to Oban. I've only cycled the latter and have to say it was fine, rolling to flat and a wide road with good sightlines and had no trouble doing a good 16-17mph even with a bike full of touring gear. However you wan't to completely avoid the A82 up Loch Lomond. It's a fast, busy road but twisting and turning so a lot of impatience for overtakes, especially considering volume of lorries and coaches on it.
Back to DaveC's route, my alternative to the A83 would be cycle out to Greenock, Ferry to Dunoon (regular, fast service) then A815 in direction of Inveraray, much quieter. Join back on the suggested route at Ardkinglas and then you've got a short, flat section of A83 into Inveraray (great stop for lunch).
Then the rest of the route is good.