What does Twain mean when he says Ben's "heart was
light and his anticipations high"? What is the impact of the phrase on the tone or feeling
of the text?

He meant that he is looking forward to all the fun he is going to have. Ben pretending to be a steamboat making sounds while tom is unhappy about the situation.

Explanation:

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.

you have to tell us what you're talking about or it's impossible to answer.