There isn't much chance of getting lost between Fort William and Oban as long as you don't accidentally head for Glencoe from South Ballachulish because there is effectively only one road! The trouble is, that means all the traffic uses it ...
On the Sustrans website, they suggest using the ferry to cross Loch Linnhe from Fort William and then use the quiet A861 on the west side of the loch as far as Corran before crossing back over on another ferry. You'd then have to do 4 miles on the A82 to Ballachulish.
I cycled from Benderloch to Portnacroish in 2011 to have a look at Castle Stalker. The parts of the cycle path that have been completed are very good, but I wouldn't riding too far on the main road itself when there is a lot of tourist traffic about.
I did an extra loop round through Port Appin on a lovely quiet little road. There is a sea food restaurant at Port Appin which looks worth a visit. I stopped for a quick coffee there and watched someone diving in to the freezing loch and going for a swim! I also spoke to a family on bikes who had just come back on the ferry from Lismore. They said it was nice to explore the island by bike.
The minor road from Creagan round the end of Loch Creran looks worth doing, but I didn't have the time. The former railway bridge over the loch is now the main road bridge, but when I was a boy the steam railway line was still in place and the road traffic had to go round the loch.
I can recommend the B845 from Barcaldine over Gleann Salach to Loch Etive. It is a lovely quiet road, and then you can take little road round the side of Loch Etive to North Connel.
From Connel, I liked the road that went SSW over the hill to the Rare Breeds Park and then dropped down into Oban.