Near Pensacola, the University of West Florida Trail System has 25 miles of trails. For great cross-country mountain bike rides, check out the Cambodia Trail, the Pate Road and Tree Hugger Trails on the west side of the power lines, which is south of Pate Road. These are fast trails with some sandy sections.
To reach the trailhead, go to 10 Mile Road (Just North of 9 Mile Road or Hwy 90), east of Chemstrand Road, and find the service road that takes you to the Crist Power Plant -- Pate Road) or park at the base of the power lines on Ten Mile Road (Greenbrier Rd). Go just past the homes on the right (the backyards of homes in the Country Ridge Subdivision). There is a dirt road which runs beside Pate Road, take this road because it leads to parking area just outside the trail head (if you are driving). There is a fence and piles of dirt blocking a dirt road -- the trail entrance is to the right of the fence which is the Pate Road Trail.
For other trail descriptions and maps, see the PORC website.
Overall, University of West Florida Trail System is the 21st most popular mountain bike trail of all 58 mountain biking rides in Florida.