There are miles and miles of Forest Service roads on which to mountain bike in the Seeley-Swan Valley, as well as some singletrack. Another option is roads that have been closed to motorized traffic but are still open to non-motorized travel.
Maps of the Seeley Lake Ranger District and Swan Lake Ranger District provide information on the forest road network as well as land ownership. Be sure to consult detailed maps well as persons familiar with the area in which you plan to ride. Please respect private property and abide by an restrictions on public lands.
Remember, bikes are not allowed in the wilderness. Practice low impact mountain biking, and be courteous to other trail users.