I second the recommendation to get your feet analyzed at a specialty shop. I'm sure that most running shops would probably carry walking shoes as well.
Here's a U.S./Canadian listing:
http://www.runnersworld.com/article/...1048-0,00.html
Also, you may want to try a few stretches before you set out on a long walk. I have plantar fascitis which causes arch pain for me and stretching before a workout has helped. I do the calf and heel stretches. Just make sure you do a small warm-up before you stretch.
http://www.mckinley.uiuc.edu/Handout...iitis.html.htm