Speed work is considered a necessity by almost all coaches regardless of whether you are a sprinter or a long-distance runner. That speed work is needed for sprinter is obvious but it is not very obvious for a long-distance runner. In my experiences working with many different levels of runners I have come to the conclusion that speed work should be placed on a continuum ranging from the sprinter through the longest distance runner.
The sprinter would have a maximum amount of speed training while the longest distance runner would have the least amount of speed training, if any. Keep in mind that the sprinter wants to exhibit the maximum speed possible in a race while the long-distance runner wants to exhibit the maximum speed endurance possible. This is a major distinction between the sprinter and a long-distance runner.
Because the objectives are somewhat different the training should also be somewhat different. The sprinter should include a maximum amount of speed work while the long-distance runner should include a maximum amount of long-distance running but run at the fastest speed to develop speed endurance not pure speed.
With this in mind if you are a long-distance runner, you should not do speed workouts only for the sake of developing more speed. The main reason for this is that the segments run are usually too short to develop speed endurance. The exact distances run should be commensurate with the distance that you run in a race. In all cases the segments run should be long enough to develop this speed endurance.
Because of this if you are a marathon runner, running segments under one mile are not recommended. Instead of only doing one mile as you develop yourself you should gradually increase the number to three, four, and more miles. Keep in mind that world-class marathoners run sub 5 min miles for the entire race. Thus not only must you be able to run a relatively fast mile but to be able to repeat the fast mile for the full 26.2 miles.
Thus speed work is needed but the form of speed work should change depending upon your specialty. Training according to the requirements of your event and not because of what everyone else may be doing. Don’t just try to be as good as the average runner; train yourself to be better than the average runner and come closer to doing what the highest level runners do.