Southwest of Tallahassee, the Tom Brown Trail is one of the best mountain bike trails in the area and is used as the training ground for the British Olympic mountain bike riders. It connects up with the Fern Trail and Cadillac Trail for a solid afternoon of riding.
While in the area, also check out the Red Bug Trail, which itself connects with a few other trails for a couple hours worth of riding.
To reach the mountain biking trailhead from Tallahasse, head east on SR 90 to Capital Circle. Turn left and continue for one mile, past the Federal Prison on the right. The park entrance is on the right.