If you want to retrieve just a single value, you should use $wpdb->get_var():
$post_id = $wpdb->get_var( $prepare_query );
But you can also use $wpdb->get_col(). Just remember — on success — it returns an array and not a string:
$post_ids = $wpdb->get_col( $prepare_query );
$post_id = $post_ids[0]; // simplified and assuming this is set