Yasso Laps is a form of interval training designed specifically for improving marathon times, it is my favourite way to prepare for a long distance event but is a punishing routine when done right. Many coaches will use this to great accuracy in predicting an athletes marathon time also, once you have the time on your feet sorted for a distance event the best way to run it faster is to do the shorter sharper interval training to build leg and lung strength.
This should be used in a periodised routine including low intensity steady state cardio technique work among other cross training.
Run 800 meters x 10 with 400 meter recovery runs in between each round.
The 800 meters should be run hard but at a pace each round you should aim to get near enough the same time the 400 meters you just need to jog at a slow recovery pace when you first do this that may be a walk but it should still be a run when done properly.
If done right your 800 meter time can be used to predict your marathon time so if you ran 800 meters 10 times at 3 minutes and 30 seconds each round then you would likely run the marathon in 3 hours and 30 minutes, this is surprisingly accurate but it works.
With only 12k of distance this is a great way to gain real marathon speed without destroying yourself with constant long distance training runs.