Here's my recommendations to link up as you like:
-Calder Glen on East side of East Kilbride
-West Highland way, starting at mugdock and on as far as you like...
-Upper part of the Kelvin, runs between Maryhill park and the Balmore road, extendable all the way to Milngavie
-Track from Carbeth over to Hardgate, crosses the bridges on Concho Loch
-Track from Baldernock Lynn over Blackskiath Muir and down past Lennox castle. (single track in the forest!)
-There is a path from Kilearn to Strathblane parallel to the A81, to the NW, sadly there about 548 gates to go through, great spot though
-All of loch Ard Forest
-3 lochs drive is superb
-Not tried it, but I reckon, you could do the flatter sections of Ben Lomond tourist path on a quiet day*
That's some of what I've tried over the years, loads more I can think of to do, I'll report when I do.
Anyone fancy some group rides?
*Bonus, a superb day from glasgow, is a cycle to Rowardennan, up and down ben Lomond and cycle home. Extra points if you manage to take a drop bar bike to the top