You should contact the enduro steward for District 36. You can also read the rule-book at
www.ama-d36.org
It's been years since I've raced a D-36 enduro, but my educated guess is they will have you start in C. If you overall the C class your first time out, they'll quickly move you up.
IMO, there is no way they will let you ride A for your first enduro in D-36.
FWIW, I know some strong, highly ranked D-36 'A' Cross Country riders that were told by the steward to ride B class for their first enduros.