I don't understand why all terrain would be difficult while prone.
PHB pg 288 has rules for crawling, and they don't say anything about that. I was simply wondering if enemy monsters would take minimal steps to preserve their dignity, like standing up, or if they would crawl towards their enemy.
Unless I misunderstand what people have said, the foes in question (under the influence of 'Come and Get it') would crawl to the fighter, unless obstacles, people, or difficult terrain made the path impossible.