Total, try the Don Valley, you can access trails at Pottery Rd & Bayview and go from there, its pretty good riding, lots of short but intense uphill and downhills.
It's off camber, you're riding along the side of the valley alot. One word of caution, keep off for a day or two after it rains. the trails are in a state of overuse, and when they get wet, they really take a beating!!
Also there are a lot of ppl hooking up for group rides some on weekends, some weeknights.
I also know that in the Humber area there are trails too, but havent personally ridden them.
Try MTBR.com, they have trial reviews too.
Good luck, and maybe Ill see you out there!