The Longleaf Trail is in the Chickasawhay Ranger District of the De Soto National Forest, a longleaf (and in some areas slash) pine forest with rolling hills and many small streams. Some fire roads, some old logging roads, and a lot of single track hiking / horse trails. Lots of shade. Much of the brush in the forest and the pine trees themselves are green year-round. The pictures below were taken in May rather than March, but will still give you a good idea of the terrain and vegetation you will see during the run. This is not a hilly course by ultra standards, and there are no rocks. Muddy, tree roots, overflowing creeks? Well maybe, see weather!  
A nice way to preview the scenery you will be seeing in the race is to take the Gavin Forest Education Tour, a 45 minute driving tour which circles the forest.  

Here an old logging railroad bed takes you across a lowland area

Here a runner climbs one of the steepest hills on the course, leveled by the camera

In 1999 and 2001 the water level at this creek was just under the trail marker on the tree at the right of the picture. Ropes were strung to assist weak swimmers. Even though the water was deep, it was clear, and the current was strong.