In reply to DR:
Some great trails round Aberfoyle and Strathyre. There's Comrie Croft which is a trail centre but a pretty rough and ready one so is actually much better as a result. You've got North Third (climbing nearby too) near Stirling, and there are trails off the top of Dumyat. I'd suggest downloading the trailforks app and having an explore. From Glasgow you're not a million miles from Dunkeld or Dunoon too. Dunoon is less steep but still technical, while Dunkeld is one of the best sets of real trails in the country.