Automate post update for all posts?

The problem is you are not “filling” the custom field with data you are just outputting the value in the edit screen, so you must save or update, and to have that data inside all of your posts you will need to either create a custom query of all the posts and update there meta or manually edit each post. Both options are bad when there are 1500 posts.

A simpler solution would be to update the data “on the fly” meaning that on your theme file (the one that displays the meta data) create a conditional check if that field exists and if not the update it, something like:

//get saved data
$page_description = get_post_meta($post->ID,'custom-field-name',true);
//check if data exists
if ($page_description){
//if it exists just echo it out
   echo $page_description;
}else{
//if not , update the post meta with your default value and the echo it out.
   $default_value="This is a sample value that will be put (echo $page_description) into a custom field";
   update_post_meta($post->ID,'custom-field-name',$default_value);
   echo $default_value;
}