As you can see, a large section of the Mayor Midnight Marathon is done on trails.
You might think all running and its mechanics are the same. Essentially, sure, it is. Left foot follows right foot follows left foot, follows right foot. Repeat that for a while. But different surfaces play a significant role, and they shouldn't be underestimated. The impact your lower body feels is directly related to the hardness of the surface on which you are running. The harder the surface, the lower the stress point on your body.
In other words, running on concrete - one of the hardest surfaces - affects your ankles most prominently. Running on asphalt - only slightly softer - will stress your knees more. Running on dirt or grass tends to affects your hips. Your ankles and knees absorb the shock a bit better, and your hips take the brunt of the work.
I've a fair amount of dirt trail running in order to prepare for the race. Many of my weekday runs take place along the horse trails at Griffith Park. Running on uneven surfaces such as these teach your ankles to react to slight changes in grades. My old soccer coach used to tell us he strengthened his ankles by running on dry river beds because they provided a stabilization workout for the feet. As one with bad ankles, I hope the trail runs have helped. But as one with the occasional bad karma run, I hope I don't trip over a tiny boulder along the way.
And, of course, there are other obstacles to watch out for. FitSugar has some advice about how to prepare for hitting the tracks.