Registration only for Great Wall of China Marathon?

So it would seem that to enter The Marathon one must book a tour package and to enter the Marathon one can enter direct and just for the run? How can I just enter one the runs without a travel package? I am currently living in Taiwan and have been lucky enough to visit Beijing before. Due to limited leave and a budget, I am not interested in an expensive tour package in a fancy hotel. I would just like a short stay in Beijing, do a run on the wall and then take a tour to see the Terracott Warriors.