I've been frustrated tonight making adjustments in preparation for tomorrow's ride. I just can't get it right!!!

So, a friend comes over and shows me that the rear shifter cable is frayed to a point of near total failure (oh, wouldn't that we fun on tomorrow's hilly ride!)

This one is DuraAce with only 1,900 miles on it

How long should a shifter cable last?

Have I done something to cause this???