You have some text with lines that are too long or too short, so you’d like to re-wrap them to be more readable.
Use the fmt command, optionally with a goal and maximum line length:
$ fmt mangled_text
$ fmt 55 60 mangled_text
One tricky thing about fmt is that it expects blank lines to separate headers and paragraphs.
If your input file doesn’t have those blanks, it has no way to tell the difference between different paragraphs and extra newlines inside the same paragraph.
So you will end up with one giant paragraph, with the correct line lengths.
The pr command might also be of some interest for formatting text.