The fastest marathon ever run was by Eliud Kipchoge, with a time of 2 hours, and 25 seconds. Kipchoge and 2 others were part of a program by Nike to break the 2 hour marathon barrier. The participants followed a training and nutrition program. They also wore special shoes designed by Nike.
But, it was on a closed course. There were other runners on the course who paced the participants, and who also reduced air resistance for the runners. The pacers would periodically trade places with rested pacers. An electric car set to 2 hour marathon pace was on the course. For these reasons, this "world's fastest" time is not a world record.
The world record is 2 hours, 1 minute, and 57 seconds, also by Kipchoge.