The fitness thing has been there for years - I remember the ABs being fitter in the 90s than anyone else around.
What surprises me, is that no-one (bar the 2003 English team) seems to have caught up.
There must be something more to it than aerobic and anaerobic fitness and strength. It is the skill whilst under that pressure (and the depth of the reserve bench) that are the differences, and that all the players, 1 through 22 are ball players, and happy to run with the ball and know where their backup is.