Hi Katy!
Thank you for bringing your question to the Disney Parks Moms Panel!
Driving to Disney Springs and taking a bus to the parks is actually a really good idea. There will be road closures and delays around the resort area until about 2:00 PM. They will gradually open back up, but if you want to avoid them all together, that is a good option.
Runners will be through
Magic Kingdom park before it opens, so that is also a good park to hit on the Sunday of the
Walt Disney World Marathon. Roads closest to that park will open back up first, so driving there would not be very difficult. It is definitely the least affected park that day.
Be sure to check at your front desk for road closure information. Google Maps and the Waze app are also good to use as they normally have updated road information and alternate routes.
I hope this helps! Please let me know if you have any other questions.