Hi RebekahM,
One of the biggest benefits of running is that it makes your body work harder than walking, thus you will burn more calories. That is the most simple way to differentiate the two.
I like what you are doing with adding the tenth of a mile each seesion. By doing so, you are raising your level of activity each session, but doing it in a way that makes it obtainable and safe. Keep doing what you've been doing!
Dreams are great and you're proving that they don't have to be out of reach! You can run the marathon and it will be more than a hoot!
Keep up the good work!
Good Luck,
Paul