If your previous data is stored in a database that can be accessed through a programming language and SQL statements, then I would definitely recommend going the XML-import way.
Otherwise, it is easier and less error-prone to insert the data manually, although it may seem like a manually intensive labor.
Writing the XML by hand is more time-consuming, requires more skill and attention to detail, than going through the quick-submit process, which is faster.
It would require basic copy-paste activities and attention to metadata detail (such as author information, and other indexing info - for authors without e-mail, use <![CDATA]> as the value for the field ).
With the newly added feature to change the date of publication manually, it is just a matter of getting the data inside OJS.
How long it takes will depend on how much historical data you'd like to add for the launch date.
Historical data can be added at any time.
Our experience shows that a single person can add a 16-article issue within a week, without pressure.
You'll need to create all the available sections first, though.
All other data is added during submission.
What will be a boring task is to download all files for a new "upload" process, or have the URLs handy during submission [usually the browser will allow you to inform the URL as the file location, download it to a temporary file and resubmit it], as the submission file's name is changed to fit OJS's naming conventions.