The North Shore Trail on Lake Grapevine is the best mountain biking trail in the Dallas-Fort Worth metroplex. The trail is an 18-mile round trip on singletrack, and 5 miles each way of the single track is one-way. Only 4 miles of the trail are shared out-and-back, making the tight turns on this trail much easier to negotiate.
Directions given here are to the MADD Shelter, which is a good starting point for both mountain biking and trail running.