Complex sentence: Early in 1936, Earhart started to plan a round-the-world flight--not the first to circle the globe but the longest at 29,000 miles (47,000 km), following a grueling equatorial route.
Split into simpler sentences: Early in 1936, Earhart started to plan a round-the-world flight. Not the first to circle the globe, it would be the longest at 29,000 miles (47,000 km), following a grueling equatorial route.