Since this is
a very hilly century, with 11k ft of climbing up to 25% grade, how should I slant my prep work, or what extras should I do to be ready for these hills? I have plenty of hills nearby to work with.
You've been given good and thorough advice, IMO. The simplest answer, though, is to follow your schedule and make sure that several days each week -- including your long day -- are in the mountains. 23% is no joke.
FWIW, I don't approve of having your long day on Saturday and then, during the event week, taking Saturday off and having Sunday as your event day. Not a huge deal, but a "sensitive organism" can take offense at such a change.
HTH!