faulty parallelism is a construction in which two or more parts of a sentence are equivalent in meaning but not grammatically similar in form.
the correct version of this faulty parallelism in the sentence is:
the hikers knew that the hike was six miles, that it usually required the entire day, and it would not be wise to start after 10: 00 a.m.
the problem is the last sentence, the first two have a basic subject+verb+object structure.
so, to give a proper balance to the structure, or better said, to give a parallel structure, we must write a basic s+v+o structure in all three sentences:
the hikers knew that the hike was six miles, that it usually required the entire day, and it would not be wise to start after 10: 00 a.m.