Welcome to the second phase of the "The Marathon Rookie" plan (8 weeks).
This "Build" plan is mainly about the building on the aerobic endurance stressor. Which is just a fancy way of saying 'improving your bodies ability to withstand longer runs. There is also a single day each week dedicated to improving your speed at VO2 max (maximum oxygen uptake).
NOTE: Although heavy intensity workouts result in quick and significant gains, they can and often do result in injury. This is why it's important to NOT skip the lifting workouts of this plan. The lifting is specifically designed for injury prevention as well as performance enhancement.