No, you are not crazy for thinking about it.

Why do you want to do the century? The challenge of completing something tough? Because it is local and you ride and you "should" do the century? If it is the latter, eh, skip it. If you are jazzed about 100 miles and looking for a challenge (and don't mind too much if it kicks your butt), then do the century.

Can you sign up for the metric and decide about 40-50 miles in to the ride?