From US highway 191 heading south toward Moab, turn right on highway 313 toward Dead Horse Point State Park. Horsethief campground is on the right just before the Gemini Bridges road.
The Horsethief area MTB trails start at this campground and are excellent to run on. There is a series of trails and loops, so you can pretty much do a run of any length you want. Nicely undulating without any steep climbs, as mountain bikers apparently don't like those too much.