Welcome to the club Pres.Merkin! As DopeyRunr said, the events (especially the 10K and 5K) sell out very fast, like in 90-120 minutes. If you are not ready to commit, you can still sign up for a race with a charity (minimum fundraising amount) or travel agent (minimum night stay and park ticket required), though those can sell out as well. Unfortunately, race registrations are non-refundable and non-transferable but the safest way might be to register when they go on sale, and then just "eat" that cost if you don't end up going.