You don't need a park ticket to do the walk itself, and you get free parking @ Mickey and Friends. To do the walk, you just need to fundraise or donate a minimum of $50. You get a wristband and AFAIK you get into the parks before they open for the day.
If you want to spend the day in the park after the race, yes, you do need a ticket, as far as I know...but CHOC Walk offers some sort of $45 discounted ticket if you're walking.
Sorry to bump this from last week, but I will be walking for the first time this year and wondered who else would be there... I joined the annual passholder's team 2012 CHOC Walk in the Park | Passholder News | Disneyland Resort
I must be honest, I'd never heard of CHOC before this, but it is a great cause and a great excuse to go to DL for the weekend!