The trail is mostly doubletrack because it's intended for ATV's. Portions of the trail are on forest roads but are few and far between. The majority of the trail is rocky and rooty with at least one designated rock park that is extremely technical. Once you cross Forest road 124 (near Pigeon) you have the option to head northwest to complete the western Marienville loop or head to the east to access the eastern loop and also the Timberline trail. The trail I did was the western Marienville loop and was about 23 miles long. The trail is fairly well marked with yellow and orange arrows with a few wooden bridges along with some shallow seasonal stream crossings. Take lots of energy bars and water. This trail is long!
Heading north on Rt 66, there is a fire tower on your right. Turn left at the fire tower onto Forest road 221. Follow this road until you come to a small cul-de-sac-like parking area. Park here. Bike on 221 to the North-east and the trail actually starts about a mile from the parking area and will be on your right. It's a one way trail due to the ATV's.
http://www.singletracks.com/item.php?c=1&i=3071