Finally for your conversion keywords (mesothelioma! :)) only value success by measuring conversions, leads, multi channel offline impact. You also map things back to the customer consideration life cycle, achieving better targeting and landing page experience on your site.
You have thousands of pages. The second source of data are the logs from googlebot that indexes the world wide web and all the pages (and your site). These logs understand what your site is, what content is on it, what you sell, service, do on those pages.
That makes sense because more people who type in key phrases associated with you will find your web pages higher in the search results and hence will most likely end up on your website. What if the third is consistently the one that lands someone on a relevant landing page?