In order to populate your Long Template Page with additional sections, you must:
1.) Create additional Sections by going to 'Additional Content' from the dashboard.
2.) Add a new Additional Content or choose existing 'Additional Content'
3.) Find post ID of the additional content. See screenshot:
4.) Revisit the Long Template page and add any of the following custom fields:
-secondary-content
-third-content
-fourth-content
Example:
Created with the Personal Edition of HelpNDoc: Full-featured EPub generator