I've never done a 12-hour race ... but I have done a 24-hour race, twice. I loved it!! I just rode it like I would ride a long brevet, but as fast as I could because it was only 24 hours and not something like a 1200K (90 hour) randonnee.

Is it the 12-hour version of one of the ones on my Links page?
http://www.machka.net/links.htm