Ah gutted! One week out. I stop work for there weeks on that Friday evening! I'd've been well keen for a pedal. Ach well.
Right, so anyway, as Andy pandy says, your closest dedicated trail is going to be Carron Valley. I wouldn't use the word basic, it's got some good stuff! Not too long that you get lost in it all and not too short that you get bored after 1 run. You could easily hit 3-4 laps and still be having fun. Just over 5 miles a lap. Have a swatch of those strava screen shots. I was riding with my non cyclist brother so don't take those times seriously the last section is a 'jump park' and you can get some decent speed up on it. There's loads of whoops on the trail to keep you pumping for speed and a good few wee drops and plenty of bermy turns on the top section.
Glasgow wise, you've got the commonwealth track at Cathkin braes on the south side. It's much the same as CV. Not a massive lap but it's great for sessioning. It's got a great wee dual slalom track and some right good flowy bits in the trees. CB I would say is a good bit more tech than CV. Look out for the dog walkers though!
If you were up for an adventure, hitting Ben Lomond would have been sweet! Might be capped in snow at the moment though. http://youtu.be/a-4DRWEadjE In the same vein, the west highland way from Milingavie > North, as far as you want to go, is very much rideable.
East Coast, you'd be silly not to try Glen Tress or Innerleithen. Google that shit.