i'm of the old school thought that you should try to train for the situations that occur in your competitions. for example, shorter faster intervals are great for getting you used to the efforts in a crit. longer, steadier efforts are usually seen in road races. a large part of this is training your mind to tolerate the pain that these different efforts cause. 2x20 is mentally difficult to do....but so is trying to do a solo breakaway for 15k.