My first charity rally was for Muscular Dystrophy. They enthusiastically helped. Afterwards, when I gave them the $400 check from the rally, they said, "We can make $40,000 from a 10k run. Thanks, but we will go back to what we do best!" So spend more time than we did on an event format that gets donations, and don't try to make an existing $25-entry-fee event get your income. That said, we do have a strong rally community not far from you. They might help. Look up Cal Club Road Rally.
I live in Manhattan Beach, CA and I'm planning a road rally to raise funds for the Cancer Journeys Foundation. The event will be next August in the northern California region - Highway 36 will be part of it.
This will be a first time for me and I would love to get your input.