It’s pretty obvious that you need speed for the 100 & 200m but many people neglect speed for the 400 & 800m. Most coaches and athletes advocate the long to short method meaning they start with very long slow running early in the season and progress to short faster running later in the season. This is a very common method used around the world and works very well for many athletes. However I would say it works for many talented athletes if they already have a load of speed. Michael Johnson, the 400m world record holder with a blistering time of 43.18 trained on this method and who can argue with his times. However he could also run a 200m in 19.32sec and the 100m in 10.09sec so this gave him a pretty big advantage in the long to short method. I would nearly go so far as to say that you could train a gifted athlete like him on many different programs and get similar results. This is not to say that he didn’t have to train hard and didn’t have the best coach in the world at that time in Clyde Hart. And I think he had the best coach for him.
I have a saying for any athlete that wants to run 400’s & 800’s. “If you don’t like feeling uncomfortable at every training session find another distance”.