Learn German: Diese Entfernung fliegen sie den ganzen Oktober.

This is the adverbial accusative of duration. In German, when you want to express how long something happens, you do not need a preposition. You just put the time phrase directly into the accusative case. Since months are masculine, der Oktober becomes den Oktober.

Does diese Entfernung also use the adverbial accusative?

Yes, it is acting as an adverbial accusative of distance. Just like time and duration, specific distances are placed directly in the accusative case without needing a preposition. Because die Entfernung is feminine, the accusative spelling happens to look identical to the nominative.

Why does the sentence start with the distance instead of the subject sie?

Moving diese Entfernung to the first position puts special emphasis on the distance itself. Because German requires the conjugated verb (fliegen) to stay strictly in the second position, the subject (sie) simply moves to the third position right after it.

Why does the adjective ganz get the -en ending here?

It follows the standard rules of adjective declension. Because the phrase is in the masculine accusative and comes right after the definite article den, the adjective takes the weak -en ending, resulting in den ganzen Oktober.