Fountain Head is a great workout. It's not overly long at 6 miles so you may want to ride it a couple times. It has a few tough up hills(nothing huge just more steep and rooty), but with up hills come great downhills.
How to get there:
From the intersection at Lee Chapel and Ox road(123)turn right off of Lee Chapel onto Ox. Follow it for approximatly 1.5 to 2 miles until you get to Hampton Road. (you will see a brown, rectangular sign that says "Fountain Head Regional Park" right before the turn.) Follow hampton road until you reach the park about 3 miles later. It will be on the left. Go down the park road until you reach the parking lot. Park your car, unload your bike, and start riding.