What's your budget? How stiff do you need?
Mountain bike shoes are designed so you can walk around in them (as opposed to road shoes, which aren't). Higher-end models can have very stiff soles.
You might also look at Shimano's "mountain touring" shoes (models start with MT-). They look more like regular hiking shoes. The soles aren't super stiff, but they're probably good enough for a century. I wore my MT-31s when I rode from San Francisco to Los Angeles. I installed a thicker insole before leaving. They were fine for the first 4-5 days (~70 mi/day). After that, I started to get hot spots after 4-5 hours of riding. Next time, I'll use a stiffer shoe. Perhaps my Specialized BG Comp MTB shoes or maybe their new Tahoe Sport shoe. The MT31s were fine for walking 5-6 miles around Hearst Castle, BTW.