At least 10 miles of singletrack, possibly more. Many mile of hard pack, with a few areas of soft sand. It is pretty flat, but has some technical areas, a lot of switch backs. The trails are marked for beginner through expert. There are a large number of tree roots, which will beat up you and your bike.
"Get off on the I-95 exit to Jacksonville Beaches. Get on to Atlantic Blvd. Go down Atlantic until you reach Mayport Rd. (about 20-30 minutes) take a left onto Mayport Rd (Mayport Naval Station). Go about 5 miles down Mayport Rd and you will see signs for the park. Admission is 50 cents per person.
http://www.singletracks.com/item.php?c=1&i=134