I use a workaround for those ‘need a flat road’ days. This procedure assumes a Windows machine. Create a custom workout of the duration you desire with ‘freeride’ as the only block. Save, then go to Documents>Zwift>Workouts. Right click on the file and open with Notepad. 90 minutes looks like this:
<workout_file>
< author>B.Kipper< /author>
< name>90 MINUTE FREERIDE< /name>
< description>90 MINUTE FREERIDE< /description>
< sportType>bike< /sportType>
< tags>
< tag name=" FREEATLAST"/>
< /tags>
< workout>
< FreeRide Duration=“5400” FlatRoad=“0”/>
< /workout>
</workout_file>
Change FlatRoad=“0” to FlatRoad=“1”, save the file, and you’ll be free to ride steep hills as flat roads. Happy riding 
EDIT: apparently xml code is filtered, so I’ve added spaces that wouldn’t normally be present in the file.