...are like this. Suppose for a moment that I am a pro video editor (: I know final cut in and out because I use it everyday. The best thing for me to do is try to edit better but faster and spend less time cutting thereby making more money for my time and having more energy to make good creative decisions. (or running quality strong miles vs. quantity of miles and focusing on recovery) If I want to improve in a certain area I can do specific training to educate myself. But any other extra time spent using the software won't really help me. If anything I'll burn out and my overall performance will suffer. However, if I want to learn a new software and maybe learn to cut a new medium like feature length films within final cut, it is good for me to spend as much time as possible doing it. Even if it's just doing the basics over and over, not necessarily straining myself. Because I will be forming good habits and developing 'muscle' memory that will be good for me in the long run. More simply though, in the case of ultra-running... it's all junk miles. The point is to go slow enough to be able to run without stopping. Inevitably you will gain fitness but not really see improvement in speed.