Hi,
I had cycled in Mongolia for two months back in 1996, and since then we collect all kinds of information about cycling in Mongolia. We also have good quality topographic maps.
There's a "main track" leading from the Chinese border to UB, and it has quite some traffic. The more interesting rides are away from that main track. So if you have enough time, I highly recommend that you spend some time for side tracks.

For information you can check out our website:
Mountainbike Tour Mongolia
happy planning,