Hi Catherine! The first trip to Disney World is so exciting! And the 2013 marathon will be the 20th anniversary, so you picked a great year!
Your daughters' trip will not be adversely affected at all! You will see a lot of runners throughout the weekend, but it doesn't affect crowd levels much and all attractions that would normally be open will still be open. Disney does a great job of getting everything back in normal working order after the races. They begin early in the morning, so even traffic is back on schedule by noon. All parks will have normal hours and buses will be delivering guests to the parks as usual. Other than seeing lots of sweaty people and plenty of medals worn, you wouldn't even know it was going on.
Be sure to check out the spectator resources that are available on the
Walt Disney World marathon website. Your girls can be inside the Magic Kingdom watching you as you run toward Cinderella Castle! Just click on the blue link and then on "spectator resources" for more information.
It will be a magical trip for everyone! You may want to plan a few hours on Sunday afternoon as a pool/recovery day for you, but other than that, you will have a great trip! I plan on being there with my boys that week too.
Let me know if you have any other questions as you plan your very first Disney World trip!!