The book linked below has a neat discussion about endurance work building endurance but hurting quickness, and quickness work building quickness AND endurance.
Someone told Herb Brooks about it, and he didn't really believe it, so he checked with Roy Griak (Griak was the U. Minnesota track coach; I had never heard of him, but I am sure a lot of you have) and Griak said it was right on. So Brooks changed up a lot of what he was doing. He went to shorter intervals with more rest, with more of a focus on guys being fresh, rather than just beating them to death.
The theory won't be new to anyone here, but it was interesting to see it in this context.
The discussion is on pages 138 & 139.