I think there might still be one or two short sections where you have to use the A828?
There used to be an on-road section at North Ledaig but an agreement has been reached with the caravan site owner there to allow access through his site to the cycle path beyond.
Benderloch has at least one village shop. The cycle path from there to Barcaldine is very good and flat.
At Barcaldine, you go 'off piste' and climb slightly round the fringes of Barcladine Forest before dropping back towards the road at Sutherland's Grove. Nothing too severe, but definitely not flat for about 3 kms.
You can cross over Creagan Bridge from South Creagan but I liked the diversion inland on a quiet road round the end of Loch Creran. It adds about 8 km to the distance and it does include a stiff little climb before a quick descent to Creagan at the northern end of the bridge.
The path from Creagan to Portnacroish is very good, but I liked the road detour to the left and round to Port Appin. I encountered a surprisingly steep little climb/descent on that road, but it wasn't long, so a short walk if you found it too hard. IIRC there was a hotel with a seafood restaurant there. You can also catch a ferry the short distance across to Lismore. I haven't done that, but when I was sitting at the jetty I saw families getting off the ferry with their bikes, having explored the little island.
From the path at Portnacroish, you get a fine view of Castle Stalker. You have probably seen it on tins of shortbread, or if you are a Monty Python fan, here...
I didn't follow the path any further towards Ballachulish (and it is a few years since I was up there anyway), so I'm not sure if the missing bits of cycle path have been built yet? (At the time, there were negotiations going on with landowners for access, and I think even thoughts of building a cantilever-supported path in places over the loch.)
PS Definitely
NOT flat, but a lovely alternative route from Connel is to head east along the shore of Loch Etive towards Bonawe then turn left and head over Gleann Salach and rejoin the cycle path at Barcaldine.
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Quite a few of my Scottish ancestors are buried in the cemetery at Ardchattan Priory.