you register at the race. can't say how long it will be - in the past it's been between ten and twenty miles - not super strenuous.
you can expect to race around to various grocery stores and buy items that your manifest. you might have to do some thinking about the best route from the options available. you'll have to carry the stuff that you buy and deliver them to the finish line. then everyone will go party.
queerpunk's right. it's the best race of the year. And I'm not saying that just because I'm organizing it.
I don't post here anymore, and it only occurred to me tonight to check BF to see if you all knew about the race. You should come out & do it. It's less competitive (for most people) and more about contributing to a good cause.
If you can't come, or don't race, consider making a donation anyway. We're raising funds for City Harvest and the New York Bike Messenger Foundation: <a href="http://www.cranksgiving.org">Cranksgiving.org</a>