Whole Enchilada Trail
This is an all day adventure that may be one of the most unique rides you can do in Moab! It starts atop the La Sal Mountains (mountain range southeast of Moab), and ends all the way down at the Colorado River. The Whole Enchilada encompasses two trails that are located inside the Sand Flats Recreation Area – The Lower Porcupine Single track and the Porcupine Rim Bike Trail.
This rides is 26.5 miles long (42.4km) and has 7,000 vertical feet of down hill!
Utah Mountain Bike Trails has a great detailed description of this trail.