The Inner Glow Way
Choose black color and adjust the
blurto your liking (make sure to change color mode to
normal! default is
Rasterize the the object (
Image Tracethe object to create shadow levels (adjust number of colors)
Tinker with it.
Here is an example:
Multiple Strokes Way
You can create the effect using multiple strokes. To add multiple strokes open up the
Appearance window and click on the
Add new stroke button / icon, it is the first in the bottom left side of the panel.
Make sure the strokes are
inside the object for optimal control.