I took a boxing class in college. Besides the fact that it was brutal to learn how to take punches, it was the most physically demanding sport I’ve ever participated in. I couldn’t believe how tired you would get fighting short one or two minute rounds.

As for football, if you are big, strong and fast, you can step onto a football field and be a difference maker for a high-level team the very first season. That immediately disqualifies it from being a hard sport. You just need to be big and strong and fast, and then it can come quite naturally (of course this depends on the position).