One short extract from a Bruce Fordyce interview in Ultra Running magazine back in 2005 ...
Mark: What are the most important things an “average “ultra runner can do to improve performance?
Bruce: Run faster not longer! When I went to Chicago for the U.S. 50-mile championship, I thought I was better prepared than my American competitors because I had focused more on running fast than longer, which of course is not to say that long runs should be ignored.Bruce ran a 4:50:51 at that 1984 AMJA U.S. 50-mile championships in Chicago.
What an endurance runner is really looking for is running efficiency. An efficient runner is going to cover distance with less effort, and probably faster. I'm adding some faster training runs (interval workouts) to try to improve my speed and efficiency.