What I like about running on trails is how it keeps you focused on what you're doing, and thus you stay right in the moment. If your mind starts to wander off into the past or the future and you stop paying attention to your surroundings, you will inevitably trip on a tree root, step too hard on a rock, run into a low-hanging tree branch, go off the trail, or any number of things. It's not that you have a constant spoken narrative going on in your head--in fact, it's NO narrative going on in your head... what happens is you start to have a "mindful alertness" where you just become "one" with your running, right here, right now. And it feels like you can go forever.
Here's a little clip of just walking on the Croom Trail; when running, even at a slow pace, you will come upon the roots and other changes a lot more quickly, so you must, literally, stay on your toes, running light and easy..
That "one with the run" feeling happens when you do any kind of speed work too, when you have a relaxed, "mindful" focus. It's that being "focused" that's the key. Paying attention in a joyful relaxed way.