It was times like this that made the old control-U cheat (to reveal the whole map) valuable.
Try going to your Program Files\Steam\SteamApps\common\FE Legendary Heroes directory, find LegendaryHeroes.exe, and send it to the desktop to make a shortcut. Open the properties of that shortcut, find "target", and change it so that it looks like this:
"C:\Program Files (x86)\Steam\SteamApps\common\FE Legendary Heroes\LegendaryHeroes.exe" -cheat
That is, add the -cheat outside the final quote.
Now, start the game with that shortcut. When you load your saved game you should be able to hit control-U to reveal the entire map. Warning, on large maps it might take a while to work.