Gig, agreed - one of the things about the writing that made the episode seem sloppy to me.
It would've made more sense if Tyrion had convinced Dany that the WW's were true, or even that Dany had a vision or dream, and decided to head out on her own and surprise everyone. But the whole "you run back to the wall! Dispatch the raven! Fly to the battle (which you have to find)!" all before the water freezes again was just lazy.
Again, I know we aren't debating how "realistic" the show is, but some things just leave too big of a gap and could have seemingly been addressed with smarter writing.