Start at the West Creek Wetland Learning Center in St Marys, PA. (Google maps snap shot with coordinates included). Run 19.5mi to the other end in Emporium, PA. The first .5mi is less maintained but very runnable. There are more large puddles and overhanging vegetation, but it's not a bad trail by any means. You'll then pass the "official" trailhead. From here on out, the trail is wide open, flat and well groomed. You'll follow West Creek downhill to the eastern terminus in Emporium. Run all the way to the stop sign, just past the kiosk, where the trail meets East 2nd Street.
I did this starting at 5am on a Saturday and only saw three other people on the entire trail. I saw white tail, grouse, turkey and a blue heron. There were also several informational boards along the trail that explain some of the indigenous flora and fauna, as well as some local history.