The weekend long run.
These will be easy runs and will steadily increase in distance. They are fundamental to building up your endurance and tolerance for the marathon distance.
A couple common questions are , "how many miles will I run? and how do I know I'll make it through a marathon if I never run 26mi?" You will max out at long run distance of 20 miles. This will give you a substantial amount of time on your feet, build your endurance, but also leave you fresh to continue training. Simply put, if you're running 26 miles in training, the recovery is too long to be productive. All the other runs that you do will help you build up the endurance and confidence to make it through the remaining 6.2 miles of race day. You also have the benefit of a taper to help your body recover and be stronger than it was on other long runs.