Soooooo Balmaha along the west highland way to Inversnaid and back is really rather good. A good mix of singletrack and fire road. The route after Rowardennan is currently the higher route. Work is ongoing to improve the low path which will make a nice ride even better. Pub stop at Inversnaid or Rowardennan or both. Conic Hill is worth doing with the path up the back completely redone last year. Cracking views!
Old school xc is to follow the west highland way from milngavie to gartness then take the back road to Killearn where you can join the water board track all the way to Strathblane then in through Mugdock.
Mugdock is pretty good if you follow your nose and take a look at all the singletrack options. There is more than enough to keep you occupied. The trails aren't that bad to be honest. I was out for several hours yesterday and I've seen them worse. Good cafe options at the visitor centre.
From left field, the Clyde walkway starts out as a ho hum paved ride along the Clyde but from Cambuslang, Things really look up with lots of really nice riverside trail all the way to Lanark where you can hop on a train back to Bearsden. A bit muddy the other day but still really rather good. Planning on doing it on the fat bike all the way to Tinto Hill this month.
Another option is the west highland way out via Conic Hill, return in a loop by going via the track from just before Milton of Buchanan that heads south to Badshalloch. Short back road to Croftamie then take the national cycle route trail to Gartness and either return via the westie or take the Killearn option above. I rode it last Aptil from the west end of Glasgow in about five hours without going super fast.
Oh and the Hillfoot cafe rocks for a post ride ice cream.