The link above is good
If you can start from Glasgow and get the early Fort William train (5.30 am) or the early Inverness train (7.06 am), you have have a large chunk of the south-west and central Highlands at your feet - Arrochar, Crianlarach, Bridge of Orchy, Cruachan, the Rannoch Moor hills, the Tilt, Drummochter hills and the Cairngorms to name some places.
Or you could try getting the train to Gourock and the ferry to Dunoon. Not sure what the bus services on the Dunoon side are like, but pretty sure there are some.
Or train to Ardrossan, and ferry to Arran. There is a busa around Arran, but also some good walking fairly near the ferry terminus.
Glasgow is pretty well connected to places you can go walking