From London it possible to get coach tickets to Glasgow and takes about 8 hours (overnight ones possible). It costs about £56 if you book on the day. I'm not sure about train or plane.
There is a train from Glasgow to the start of the route at Milngavie and costs about £2.60. (most people start at Milngavie and walk North).
Returning from Fort William to Glasgow there is a coach run by Scottish Citylink. Ticket are usually £20.50 but if you book over the phone in advance there are fares for £5-6: http://www.citylink.co.uk/