Glasgow to Fort William
I took train to Fort William and cycled back to Erskine (Renfrewshire) in January. If cycling from Glasgow then There is a Sustrans route to Balloch (Route 7 / 75 I think) and beyond. From Balloch there is a 15 mile stretch of this track running alongside Loch Lomond taking you to Tarbet. If you're want to make good time you could cycle this bit on adjacent (fast) road. After Tarbet I think you're on the A82 all the way to Fort William though check if there are any more cycle tracks. You can take a brief detour of about 4 miles via Glencoe village to Ballachullish. All in all from Balloch onwards it's a lovely trip. Going through Glencoe is awesome (if it's not raining and you can see the hills).
Good Luck. Ron.