The NCN from Johnstone to Greenock is fine - decent surface, and flat - it's an old railway route.
If you’re feeling brave, you could leave the cycle path just after Kilmacolm and take the the track from Auchenfoyle to Loch Thom that runs along the south of the Gryffe reservoir. I wouldn’t do it on a wet day, or on a “best bike” - but I’ve done it on a road bike with 28mm tyres. I’d describe it as a “land rover track”. That would save the descent down into Port Glasgow, then the slog back up the hill.
Great descent down into Largs.
If you then wanted to add another 10 miles or so, ignore the Haylie Brae and keep going south through Fairlie, then follow the moor road over to Dalry The photo on my profile over to the left was taken at the top that hill. From dalry, either follow the main road to Kilbirnie, or pick up the NCN7.
If you’re feeling brave, you could leave the cycle path just after Kilmacolm and take the the track from Auchenfoyle to Loch Thom that runs along the south of the Gryffe reservoir. I wouldn’t do it on a wet day, or on a “best bike” - but I’ve done it on a road bike with 28mm tyres. I’d describe it as a “land rover track”. That would save the descent down into Port Glasgow, then the slog back up the hill.
Great descent down into Largs.
If you then wanted to add another 10 miles or so, ignore the Haylie Brae and keep going south through Fairlie, then follow the moor road over to Dalry The photo on my profile over to the left was taken at the top that hill. From dalry, either follow the main road to Kilbirnie, or pick up the NCN7.
It was the best of road surfaces, it was the worst of road surfaces, but that's everywhere in Scotland at the moment I think. The view over the Clyde before Greenock was great, snow on the mountains, and coming down the glen towards Largs I could have been in Switzerland. Ran out of time at Lochwinnoch so had to get the train to Paisley and cycle to the Renfrew Ferry to get home. Will definitely do the Old Largs Road again, maybe not into a headwind though. 

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(hybrid or mountain bike only)