Lava
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The distance between cities is chosen so that it takes the hero more than a week, but less than two weeks to travel between them. This avoids the need to place a "border monster" to prevent computer opponents from harassing with attacks during the first two weeks.
However, on the last map I tested, I couldn't determine "how big is the fish": I started with "improved dragonflies," and the hero had a huge movement range, allowing him to cover the entire distance in less than a week in both cases. I'll keep working on it...