Question about Using a Standing Desk

After reading all the articles on here, Lifehacker and the New York Times, I got a standing desk a few weeks ago. Actually, it is just a kitchen island table I got from Ikea, but I love it and it works great for me. I feel more energetic, clear-headed and my back doesn't kill me anymore.

There is only one problem: My feet kill me. Even after a few weeks of it, my heels hurt like anything. I generally work from home, so I prefer to be barefoot if I can, but perhaps I should get a mat to stand on? or purchase some running shoes or something else super comfortable.

Any advice?