I've always used Google Analtyics for my sites, but for one however I've forgotten to set it up.
Does Drupal have any built-in analytics, i.e. a module that comes with Drupal core, that I can look at just to get an idea of traffic volume?
The only module that comes with Drupal core is the statistics
module. And by default it is normally turned off. To use it, you have to enable it. So if it was not enabled initially, you will not be able to get the stats for dates prior to the date it was enabled.