You've done well to get that many miles out of a tire. I'm betting you had to replace the rear sooner, and as TG said, at that point the front should have been rotated to the rear and the new tire put up front.
I'm a pretty big fan of the gatorskins for both my tandem and single bikes, but I always make sure I have the newest tire up front because they are prone to side-wall failure as pictured when they start getting a lot of miles on them.