Hi Claire!
The Disneyland Half Marathon will start close to the entrance of the Disney Grand Californian Resort. I looked at a map, and it looks like your resort is about 1.5 miles to the starting line. Since there will be traffic, it may be easiest for you to just walk there anyway. The walking time was listed at around 16 minutes. That will definitely be faster than driving, parking, and then walking!
When you made your reservation, did they say if your resort was considered one of the Disney Good Neighbor Resorts? Many of these will offer free bus transportation to the starting line, so that could be an option for you. You can call 407-WDW-IRUN to find out, or even ask at your resort once you get there.
Either way, you will be pretty close to the start! You can also find a map of the course by going to the
Disneyland Half Marathon website, and clicking on "event details" and then "course".
Let me know if you have any other questions!