What no one told you was that you need a platform and you need to start working on it now. Not after the book is published but now. You don't wait until the edits are done, you do it now. As soon as you chose to go down that writing path, you must begin building a platform--which is now.
Don't go wild and add everyone you know to your new email list... there are rules and laws around this practice and you could find yourself in receipt of a huge fine... and I mean HUGE!