Great, Joe! I'd like to try 300+ km event too (e.g. Tokyo-Itoigawa), but right now I have other plans for May 19th.
I signed up for a 300 km brevet ride on May 19, 2012. That's the same day as the Tokyo-Itoigawa Fast Run (東京→糸魚川ファストラン), which consequently I won't go for.
300 km will be almost double my longest run so far (156 km), so I've got some training to do over the next 10 weeks. I intend to do a 200+ km training run from Machida down to the coast, over to Odawara, Gotemba, Yamanaka-ko, Otsuki and back to Machida as soon as possible (i.e. when it's not pouring down). Given that I managed almost 80% of 200 km at the HFC Miura ride (including a 2 hour lunch and boat yard visit and later a stupid bike crash) I think that training loop will be doable.
I already ordered a dynamo headlight because the first 6 1/2 of the up to 20 hours are going to be at night (22:00 start, 04:33 sunrise).
I'll post it here and in the rides section, there might more of us. Mostly I'm just waiting for better weather. It could be as early as coming Sunday if the forecast improves.I am busy on the brevet day, but give me a heads-up when you plan to do the training ride ~ I might be up for it too.