I could have been clearer. The answer is no, you don't have to spend an action to maintain it, precisely because it's not in your control like a summoned monster/elemental, etc. Just remember it deviates each time you roll initiative until it hits solid terrain and stops, or goes off the board. Depending on the deviation, it could be on the board the whole game. Also, be careful you don't catch your own troops, although that is half the fun!