The first 10 miles go thru a lava field making it the toughest part. Beyond there is a picnic area with water and smoother riding and easier. The whole ride is in old growth forest and the water is crystal blue. Waterfalls, pools and log bridges (walk across) and the forest make this the most fun and most beautiful ride I have ever done. Bring a camera. Difficulty is due to length, not climbs.
Above Clear Lake on the east side of the road is parking and the trail start. Starting here makes almost the whole ride, downhill. Most do this as a shuttle from there to a mile above McKenzie Bridge.
http://www.singletracks.com/item.php?c=1&i=1163