Creating marble repeat pattern

To create realistic marble with Illustrator, you'll need to use raster effects. You could layer paths and apply varying levels of Gaussian blur (Effect > Blur > Gaussian blur) to make the texture that you'd use in your shapes. This would be the most tedious approach.

You might be better off using Texturino from Astute Graphics. Then you could make your shapes and fill them with a texture resembling marble.

Or you could use Photoshop. It has a new Pattern Preview that could help.