I did a metric century (and then some) yesterday. Longest ride I've done all year. Towards the end, somewhere in the last 7 miles, the ball of my left foot started hurting like crazy. I've had my feet feel stressed while riding before, but this was a little nuts, and it still hurt off the bike.

WTH is that, and how would I avoid it?

I was wearing softer mullti-surface shoes with SPDs, much like this pair: http://www.teamestrogen.com/prodLG_1487107.html

Been wearing these shoes for years. Got the bike fit when I bought it earlier this year, and I've been ok up until now.

Very unpleasant, and if I felt that at mile 60, what's going to happen when I do my next full century?