The Fartlek/Speed Run
This refers to runs that are often classified as higher intensity. I put this one last because I do believe that this should be done to complement the overall run program as opposed to being considered the most important.
Begin with sessions that are built around time/duration and not on a fixed distance or pace. As fitness improves, workouts can be formalized with distance repeats at a set pace. Forcing high paces too early can often put you at risk for injury with little additional upside in fitness. The key is to train with the fitness that you have, and not the fitness that you want.