These trails lie on the NW facing slopes of Bogd Uul between the Yarmag Bridge (over the Tuul River) to Nukht Amaralt (camp). This is a seven square mile area (18 sq km and 4500 acres). The trails are steep two track and singletrack, heading pretty much straight up and down the mountain slopes for 2-3 miles. Much of the riding is very strenuous on 20-40% slope. These trails are not for the fainthearted. They make for great hill training and workouts. You WILL get stronger trying to ride up these slopes, and you will have fun (at least going back down).
I suggest that you start from the Nukht end (see directions) and work your way SW to NE across the system. There are four main trails that head up the mountain generally NW to SE. They can be quickly connected along the base of the mountain. And on top of the mountain with a little navigation a couple of trails can be connected, using a short portion of the Zaisan West Ridge Trail.
There is quite a bit of development that has been occurring over the past 10 years or so at the very bottom of the mountain along the main road to the airport. There are many interconnecting fences here that can make it a bit difficult to exit the system. If you start the day getting dropped off on the Nukht end by a taxi (or very nice friend), then once you exit to system at the end of the day, it is an easy road ride home of varying distances, depending on where you live in UB.