You could hit one or more of the Dumfries and Galloway trail centres – Kirroughtree, Dalbeattie, Ae, Drumlanrig from Cairnryan and then pick up the A77 or M74 depending on which side you visit.
Just in the north of Glasgow there are trails in and around Mugdock park and a loop incorporating part of the West Highland Way. You could detour via Gourock and take a ferry to Dunoon where there are a load of trails, many new ones for the upcoming Scottish Enduro race.
On route you can get to Ben Lomond and Conic Hill. There are also a couple routes from Arrochar on the A82/A83 or east of Loch Lomond traveling through the Trossachs there are lots of options around Aberfoyle and Callander.
A couple of options at Bridge of Orchy then loads again at Glencoe and Kinlochleven. Too many options to list really, depends on the route you want to take to get to Fort William and how long you want to spend getting there.
Have a look at http://www.mtbtrails.info/Trail_main_page.aspx and if anything catches your eye you can ask here for any further info.