See if you can make contact with the Outdoor Recreation/MWR folks on the local bases on Oahu. Military members have connections all over the world and tend to participate in these events more than your average Joe. They also get lots of visitors when they're stationed in Hawaii!
Also consider making a Facebook page--If you can get folks "Liking" your event on Facebook, you'd be amazed at the results. There's a big race coming up here in the Tampa area this winter called the Tough Mudder and Facebook has done amazing things for that race's popularity.
Good luck!