Ride up Sourdough Canyon Rd. It is fairly smooth and gradual. At about 4.8Miles you will come to a bridge and a trail off to the left. Go straight, over the bridge. At 4.95miles you will come to a fork. Go to the left towards Mystic Lake. At about 9.96 Miles you should come to another fork. Go to the left and down. A few hundred yards at 10.05Miles you come to another small fork. Go to the left and down below the dam. Cross the creek and go up the other side to the trail intersection. The intersection is at about 10.21miles. Go to the left and up (Toward Bozeman Cr. Trail Head) to the south-west. This is the beginning of the single track. At about 10.5 Miles you should come to what locals call "The wall of Death". This wall isn't so bad. At about 13 Miles you will come back to the bridge. Go right. At about 18Miles you should be back at the trailhead.
Considerations:
There are a unusually high number of people to crash on this trail for seemingly no reason. I am one of those people. I had a bad crash on the single-track and I don't even know how. Some people say the area is cursed by Indians. I'm starting to believe it. Be careful.
There is a forest service cabin near the lake that you can rent.
Have fun!
From Main St. Bozeman. Drive south on 19th Ave. At 5.09Miles turn left on Nash road. 1.7Miles turn right on Sourdough Canyon Road. At 0.95 miles you should be at the trailhead.
http://www.singletracks.com/item.php?c=1&i=2157