The San Antonio Road Runners put on a great trail race in San Antonio in McAllister Park by the airport in March. It used to be held in May, but that must have been extremely hot.
The 10-mile loop course is a windy maze of mostly singletrack mountain biking trails, with about 3/4 of a mile on top of a dam/flood control structure that is completely exposed. The rest is pretty shady. 50K runners do three loops. The course has mile markers, which is a nice touch for a trail race.
There isn't a lot of elevation change, the trails aren't too technical (except for some roots), and it is very well marked, so this is an excellent course for beginners to trail running or for a first ultramarathon.
After the race, there's a great cookout with fajitas and kegs of Alamo Ale - can't beat that!
