From the patch notes;
4 hours of existing flight time in a non-Discovery flight (free flight, landing challenges, bush trips, etc.) are required to display the event on the homepage, which replaces the Discovery Flight shortcut.
Though I believe the Free Flight is not tracking correctly for that, or it refers to something other than starting flights via the World Map. In my experience you need 4 hours cumulative of landing challenges, bush trips, and/or flight training before it adds the Miramar Landing Challenge.