There are between 3 and 3.5 miles of trail built out; a nice mix of fast-drying sand and interesting terrain, with more being built all the time. Check out https://www.madcitydirt.com/index.php?topic=4304.0
For directions, Googling the Prairie du Sac VFW Memorial Park or the Alliant Energy Dam at Prairie du Sac should get you here without too much trouble. The VFW gravel parking lot is at the north end of the campground, and blue signs state that it's for mountain bikers to use. It's OK to open the gate and drive in if it's closed.