The red line is the average speed of the 2012 Olympic 10K Open Water Swimming Gold Medallist. Her name, because I'm sure you've never heard of her, is Eva Risztov. She is from Hungary.
Here is a photo of 27-year old Olympic Champion, Eva...
Here is a photo of 64-year old Diana Nyad...
Both look amazing, don't they?
From the graph above, Diana, after her first 27 hours of swimming, swam for a period of about 15 hours at or better than Eva's average speed (which she only managed to sustain for 2 hours).
Here is a video of Eva and her Olympic friends swimming at 5km/hour.
Here is a video of Diana during a period she was swimming at a speed of 6km/hour.
There are two possible explanations:
1. Diana cheated.
2. The Gulf Stream is awesome.