Make it 4 the first year to make it easy, that is still a week of time for most.
6 hours for fast, 10 for slow riders. Tough on distance which also breaks bikes, too tough on terrain (1st gear stuff) isn't for big bike rally. Sand dunes are easy, BTW.
Just make the navigation standard and don't spin people around like on practice rallies. The goofiness will just cause following and make it difficult for those that can navigate to navigate away from a follower.
You should practice before you go to an event, or at least that is how I look at it. Even HP isn't difficult, its just knowing a compass heading and mileage? Right?
Bivouac, but showers are nice too.
Good luck on this, seems like there is plenty of interest when people are typing but come time to put up and show up, just the core come out.