The greenway closest to my home is about 3 miles in length, but future construction for it plans to extend it many more miles (eventually putting it right by our house, actually; but who knows how long that will take). For now, once you get to the end of the road, it just sort of dissolves into a worn, dirt path.
The path isn't very pedestrian friendly, and is probably used by more deer than humans, but it's passable as a running trail. Almost always, I stick to the asphalt, but occasionally, I've gone "off road."
It doesn't go far (about a quarter mile and then abruptly ends), and it's not groomed at all, so it slows the running down considerably.
Still. Sometimes a runner has to run off the beaten path. Just a little.