The Science Shows That Training at Low Intensities Improves Performance

The consensus seems to be that running your regular training runs at easier efforts is paces similar to ultra marathons longer than 50k may not be very useful as a physical development stimulus. For many athletes, it will work pretty well to simply keep blood lactate levels below 2 millimole. 72 to 95 percent of the miles run by high performing distance runners is at an intensity where blood lactates would not rise appreciably above resting level.