On the subject of doubles, Meb. generally just runs once a day, and cross trains in the afternoon. He also does a lot of gym work and foot speed work.
So it is possible to be world class and run just once a day, but you still need to be active in the afternoon with different combinations of cross training. It is mainly about cardiovascular fitness.
I am convinced that we will see more and more cross training replacing second runs as the years go by. Not practical for Kenyans and Ethiopians who lack the gyms, but for N.Americans and Europeans, it will be easier to gain fitness and avoid injuries in this way.
ghost