I don't run strictly on trails, but I do run regularly on trails and its my favorite kind of running!

I don't really have advice other than be certain to know the trail. Nothing stinks more than thinking you're going for a 4 mile run, get lost, and end up doing 10 miles (I've done that).

I always bring water trail running. ALWAYS (JIC the above happens).

I usually carry a whistle too. For emergencies. The trails I run on, as you can probably gather, are remote with little to no traffic.

Expect slower pace times. And expect to get really strong!