For ALL the 411 on the huge Stowe network...talk to the Boys at Irie Cycles.
They are working hard to legalize allot of trail. There is an awesome legal network behind the shop...the other area's are misty.......
Stowe/Waterbury is my Favorite place to ride...local knowledge is key.
I used to train (run/ski) up at Stowe when skiing for UVM, and there is a plethora of trails to ride. Although I didn't ride up there at the time, the trails seemed awesome for biking. I am fairly certain the last 10minutes of pushing the bike up to Mt.Mansfield will be worth it as well.
For legalized trails, have a look at the Catamount Family Center. It's not huge, but the trails are good.
Riding on Mt. Mansfield is illegal. The map you have is garbage if your looking for real MTBing. The Stowe MTB club is working hard to create legal riding in town. Right now, 95% of the riding is on private land and no map exsists. 100% of the good riding is on private land and the only way to find it is to go to Irie Cycles and ask about hooking up with a group, or head out back and explore. They are solely responsible for creating and maintaining singletrack in town, all the other bike shops are pretty much clothing store tourist traps with bikes in the window.