I found this awesome map of trails in the Uxbridge area:
Shows parts of Durham Forest, and how they connect to many other trails and areas, including parts of the Oak Ridges Moraine Trail and the Trans Canada Trail.
Speaking of the Trans Canada Trail, here are some Durham-area maps:
We rode the Greenwood section and the parts to the north of it over the weekend. If you plan to try Greenwood, avoid the northwest corner like the plague... leash-free area!
For more specific maps of Durham Forest, you can't get much better than Mike's Maps... especially if you're on a bike.