title_body: Sub-chapters based on h2 tags in ePub file
upvoted_answer: Calibre editor has a graphical interface that lets you edit the ToC of a given ebook file (the ToC editor itself is accesible also from the default window, without the need to open the full editor).
  
  You can select different ways to create the ToC, I think that in your case you should experiment by selecting "major headings" or "all headings".