Sorry if I misunderstood your objective. Adding the <guide> section makes an existing TOC accessible in a MOBI file. For example, with the Kindle, it enables the 'Table of Contents' feature when you press 'Menu' in a book. Calibre uses the TOC.NCX file to specify where the chapter breaks will occur on the Kindle.
Generating the actual TOC content is a different matter. Your question suggested that you wanted to use an existing TOC.