I've done that route many times over the years, including this year, on runs from Paisley to Balloch. The cycle path from Bowling is pretty well marked as you get into Dumbarton, but there are a couple of turns where you have to be careful to follow the signs. I normally turn north just before I get into the centre of Dumbarton on the way to Balloch, ride up to and cross over the A82 (you don't want to end up on this road), then turn left and join the road through Jamestown into Balloch. Coming back, I take the path along the Leven until I get into Dumbarton and then pick up the cycle path to Bowling again. Getting through Dumbarton can be a little tricky, but the best way to pick up the Leven when heading to Balloch is to follow the signs for Cardross / Helensburgh until you cross the Leven and pick up the cycle track on the western bank of the river. Coming back, leave the riverside at the first main road when you reach Dumbarton and cross the river again. The route is signed and skirts the town centre.