Did splits at a party at 48. Dumb. Now really hurting. What can I do?

Ice, NSAIDS. You probably stretched the tendons at your hip joint and they are telling you by the pain. Icing the area in the first 48 hours is helpful and the use of nsaids like Ibuprofen or Naproxen can be very comforting. Your tendons will recover unless torn. The sign of tearing is bruising in the area but if that does not occur, recovery should be complete. Otherwise see your family doctor for help.