Middle distance running involves a combination of speed and endurance. The races are usually too far for sprinters, but too fast for distance runners. Most of the races feel like an all out sprint, just covering a greater distance.
The runners who race these distances, need to have a great amount of running strength to compete in these events. Interval training is a great way to gain this type of strength. Interval training will build lactic acid buffers in your legs, allowing you to maintain the high speed over a greater distance.