Txt files have a fixed line length, with a line break at the end of each line.
It is easy to change this into a reflowable doc.
There are several ways.
1. Paste the text file into a doc and run stingo's word macro, which you can find here:
https://www.mobileread.com/forums/showthread.php?t=8793
2. Try putting the text into BookCreator and tidying it there very quickly. It is here:
https://www.mobileread.com/forums/showthread.php?t=28313
3. Run it through Gutenmark (google to find it).
3. Or do a search and replace:
I paste the text file into a word doc, then click on "edit" and open "Find and Replace."
1. search for ^p^p. (Click on "more" and "special" to find the paragraph symbols.) Replace with ## or @@. (This is a placeholder for the paragraph breaks. You want to strip out the single linebreaks but leave the genuine paragraphs.)
2. Search for ^p. Replace with a line space.
3. Search for ##. Replace with paragraph mark.