It is important that your body adapts to running at your goal pace before you attempt to sustain that for 26.2 miles. If you’re trying to achieve, say, a 3:40 marathon, your body needs to get used to running 8:23 per mile.
A simple way to gain efficiency is to do a few goal-pace long runs during the latter half of your training. Seven weeks before your marathon, you might replace your slow long run with a workout consisting of a four-mile easy warmup followed by eight miles at your goal marathon pace. Two weeks later, repeat the workout, but add two more miles of goal-pace running. Finally, three weeks before your marathon, do a 16-mile long run, with 12 miles of goal-pace running.
… macht irgendwie Sinn!