In reply to notaclue:
With respect to speed training, yes, there is a benefit. It depends a bit on your definition of speed training, but any type will have some benefit really. Even short sprints, which may at first not have much to do with ultras improve running form, which will help save energy in the end. Also, the higher your top speed, the higher your comfortable plodding speed. Off season is a good time to do shorter faster stuff to get some speed.