Well 3.5 hours entirely within Baldpate would be tough. You can do the whole park in like an hour. You'd be riding the same stuff over and over and likely bored out of your skull. The place isn't the most exciting place to ride to begin with.
I like to mix it up with a few other trails that are close by for variety.
Here's my ride from last week. 2.75 hours. Although there is a little too and from on the road because I ride there from my house.
http://connect.garmin.com/activity/546779185
I can assure you that you will not be averaging 12-13 mph if you head up in to the Water Co trails. 8 mph is blazing fast in that place. It's Tight, twisty and very technical at times - Boulders, Rock gardens and 90º turns.
Here's another similar loop starting from Lambertville and also including Wash Crossing:
http://connect.garmin.com/activity/365153719