I believe it was designed primarily for biking, and is maintained by local mountain bike clubs and racing organizations. If you want to make it a long, meandering run day, you can start here and then take the main trail under Braddock Road to the Wakefield side, past the community center and onto the power lines and mountain bike courses that crisscross and climb significantly in places. Some friends of mine have also extended the main trail by exploring a section that veers off from around the dam and marina area. If you want to access the bike trails from the marina, you will want to cross the dam. When you begin the steep climb on the pavement, look to your left, and you'll see the even steeper wooded trails.
Distance is truly irrelevant here, as the terrain provides enough challenge.
One word of caution, though. Be sure to exit them and return to the relatively tame main trail before you get too tired. If we are courteous, it is a wonderful resource that can be enjoyed by all.