The Lake Martin Ultra Fat Ass, or LMUFA (colloquially pronounced, "El MuhFah") is a small, private, event. If you can access the Facebook link, then you should have no problem registering.
This is a VERY FLAT trail(ish) 5.1 mile loop around Lake Martin. Half of the loop is gravel/dirt road, and the other half is grassy hiking trail. Figured this should result in some really good 50k PR times.