I'd stick to Arrochar. Yes a really pleasant train ride goes there, but busses stop too. I returned from my East-to-West Cowal Way traipse to Arrochar and got the bus home.
If you're looking for a bit of extra challenge consider bagging the Corbett
Beinn Bheula along the way, three extra miles but an extra 500 meters so nice wee detour, worth doing if the weather will give you views.
- Extra Detour