Eliud Kipchoge is the greatest marathoner ever. He’s the first and only person to break 2:00 in the marathon (it’s not the official marathon record, but he did it).
The following short video is of a “Fartlek” workout he completed about a month prior to the 2017 Berlin Marathon. You could & should do this type of workout. Not at Kipchoge’s pace, but at your pace and at the appropriate place in your marathon training plan.
Typically Fartlek is shorter distance than the 3k that Kipchoge completed. I would categorize this workout more in the strength training/interval phase of marathon training. These type of runs are repeat mile – 1.5 mile. I recommend this workout at about 30 – 45 days out from your race, you would be completing these longer intervals at race or slightly faster pace.
Fartlek workouts are typically near the beginning of your marathon training, because they allow you to work on your speed prior to getting into intervals. I recommend combining the fartlek workouts in the same mesocycle (phase of training) as hills & shorter tempos (15-20 minute brisk pace running).