Hi Elizabeth!
You will not have to pay for parking. I have driven myself to the start before, and while it isn't as convenient as taking the runDisney buses, it is not bad.
Your biggest priority should be leaving really early. I like to be parked no later than 3:45 a.m. The traffic really picks up after 3:30, so I like to be ahead of that. I also like to have plenty of time to get oriented and stretched before the race. The weather is looking fairly warm for race weekend, so that will be a good thing at 4 a.m.!
You can find specific driving instructions on the
Walt Disney World Marathon website. Click on that link, then "RUNNER INFO" tab, then "Official Race Program" tab. The transportation information for drivers is on page 56. You can download those instructions, but they will also be available in printed form at the runner's expo.
I hope this helps! Please let me know if you have any other questions, and good luck at your race!!