The marathon at
Penn State is an excellent model - the largest student-run philanthropy in the world raising $3.5 million each year for "The Four Diamonds Fund" for kids with cancer.
But it didn't start that large. You might be interested in reading its
history . Also, you could probably contact someone from Penn State directly now that this year's marathon is history. I'm sure they would be delighted to talk to you about it, talk about what goes into organizing, etc. - they've got the whole thing down pat - organizing committees, kick-off dinners, the works.
It seems to me that a key aspect is strong campus-wide support for the charity - the first few years Penn State's charity varied, but once they focused on Kids with Cancer at Hershey Medical Center (translate - kids on our own state with cancer), things really took off. It's easier to raise money when people know where it's going, know it's going to something local, and can feel that they're making a difference. So perhaps a literacy organization in your state? An after-school program in a local city?