Wow, great advice, everyone! You almost want to make ME do 500 miles on my fixie (but not quite ).

I'd second the recommendation to be prepared to change gearing based on terrain... Check out detailed elevations of the route, and maybe make a plan. I know some company (Hozan?) sells a combination chainwhip/lockring tool that could be handy. With that and a few cogs, you could have a good combination of gears available to you (and if you slip a 20t freewheel into your bag 'just in case', we won't think any less of you).

Good luck!

peace,
sam