In reply to Marq: I've not done it for a long time, but I can't believe the Citylink bus timetables have changed that much. We used to regulaly use the bus to the Arrochar region, southern Highlands (Beinn an Dothaid, Beinn Dorain) and even Glencoe. We were interested in climbing routes, but just for hill walking you will have all the options around Tyndrum as well.
It helps if you are in the West End, allowing you to get on the bus up there.