“A shadow on the sun” - When Florence wakes up and discovers she actually took that one-way flight she recalls her childhood memories of Beryl Markham and Amelia Earhart.  She recalls the morning she discovered she would never be a pilot (a leader) when she learned that her vision was flawed.  She was diagnosed with astigmatism.  Her eyes would only get worse.  There was no other way around it.  She would have to accept her poor vision and find another path in life.  (Similar to a pilot having to re-route because of technical difficulties.)

Bad vision is figurative.

“Six and a half hours” - Refers to the time it takes to fly to London from New York on a jet airplane as shown in the promotional movie for jet clippers, new jet airplanes that began to replace propellor driven short haul clippers in the late fifties and early sixties.  This began a revolution in the air travel industry.  Flights would get cheaper and faster.