Laggan black route is the best trail centre ride in the country, and the loops are short enough that you’ll be able to have a couple of goes and still have enough energy to ride the other loops. I would recommend doing the old red, then the black, then the new red.
If you’re feeling more adventurous depending on the snow Ben Lomond, the most southerly mountain in Scotland over 3000ft, is within 2 hours drive from Edinburgh. I’d try and find a webcam or some local opinions for conditions first though.
If you’re riding near Edinburgh and set on a trail centre I’d do Innerleithen rather than Glentress. Bike hire is available there too.