Or there are a couple other methods to the madness if you want to try it a different way.
Let's say your swoosh is a custom shape. From the path menu, right-mouse click it, and hit "Make selection." Then you will be able to adjust the feather radius to your liking (probably somewhere btwn 15-30 for you), and you should see the ant trail surrounding your shape.
Next, go to the Layers tab, and press Ctrl+n for a new layer. Then press alt+backspace (make sure to press d to select default colours) to get the black fade. Finally, you can hide the shape layer... and now your have another way of doing it!
Here is something I found on YouTube.