Originally Posted by jackie_w
Rather than typing all this into Calibre you may have more joy with the following approach.
Edit your source .TXT file's headings using Markdown ...
This is definitely the easiest way to do it. If you have a text editor which can manage multi-line regular expressions then you can do it with one search/replace. Alternatively you can semi-automate it using a free editor such as Notepad++ by using an Extended seach to find \n\nChapter_Name and then invoke a Macro to insert the ## markdown characters. It only takes a few seconds per chapter and it's worth the effort.