The weekly long run builds aerobic endurance, but it can be done in several ways. The choice depends on your goal race and on how much other hard work you do in the week.
Steady long run: the default; easy, conversational effort throughout
Progression run: the last 20-30% gradually faster, ending around marathon pace or a little quicker
Fast-finish or marathon-pace segments: useful in the final 6-10 weeks before a marathon, but they count as a quality session
A common guideline is that the long run makes up roughly 20-30% of weekly volume, though this depends on total mileage