Full distance (6)
(Other definitions for extent that I've seen before include "The size or scale of something" , "Area of coverage" , "Range or scope of something" , "Compass, scope" , "Degree or spread" .)
(Other definitions for length that I've seen before include "Extensiveness - measure" , "Duration, extent" , "Measure of distance, end to end" , "A brief swim" , "Object's longest dimension" .)