Oh, don't worry. I get it now. Intention like that is perfectly fine. A game that simulates realistic zombie outbreaks being difficult makes perfect sense.
But to answer your first question, I wouldn't worry about the it being too difficult. Be more concerned about good execution and making it a good game. It doesn't matter how hard or easy you make it, if you don't do it well. Give some of the readers some credit, too. Theoretically, if it's good, people will rate it well. So, I suggest making it as difficult as you want. Just do it well. Remember, some of the worst games I've played on this site has also been some of the easiest.