2700 miles / 24 days = 116 m/d
only swam 19 of the 24 days
6 h/d * 19 d = 114 hrs, or about 24 miles per hour
good swimmers can maintain 3 mph
When she said she swam "across" the Atlantic, it didn't mean she swam every yard of water from land to land. It meant, she swam in the ocean in a large submerged cage towed by a boat.
She didn't intend for the story to imply she swam the entire distance (although she traveled the entire distance, partly swimming and mostly riding in the boat). The event could be described as "daily swim sessions in the ocean as she rode across the Atlantic."