if this is truly your second race - then according to our local customs here in NC you should definately compete in the beginner class - unless you have lots of experience/are damn good and just have not chosen to race before. nobody likes a racer who drops down a class on purpose just so they can dominate - but it is also equally inappropriate to race singletrack (where passing is difficult) if you will likely be much slower than the rest of the pack.
go with the beginner class and see how you do
if you feel like you are way ahead of the others, bump yourself up next time (but don't count on dominating even this class, as there is always some jerk who has been racing for years who runs in the beginner class just so they can blow everyone else away).
being worried about what class to run in is not silly - it just means you are a good sport who wants to have fair competition.
good luck
let us know how it goes