Should you run different distances each run to improve?
I think the way to think of it is that a race distance requires different mechanisms of fitness, energy systems, if you will. Let's say you want to run a good mile. It takes systems A, B, & C at 20%, 60%, and 20% respectively. If you keep running a hard mile every day you are stressing system B to a high degree but the other two are not being maximized. Now let's say an 800 depends on system A for 60% and a 3200 depends on system C for 60% with 20% for all other systems involved. By running an occasional 800 and an occasional 3200 you can better enhance systems A and C. Even though these only account for 40% of your mile performance, by enhancing them you are improving your overall composition of mile fitness. Training and racing variety helps to stimulate certain things beyond what staying in the same groove does.