Shadow1000 said the same kind of thing about the 1000 rats goal and I meant to mention something then but forgot:

The goal of 1000 might seem too tedious to try for when you are only gaining like 10 rats per breeding and then losing 3 or so to traps etc. but the gains increase exponentially so, roughly, it takes about as long to get from 100 to 500 as it took to get to 100 and then only about half that to get to 1000. So, by the time you get 500 you'll be gaining well over 100 rats per breeding. I think I jumped from 800 odd to over 1000 in my last breeding and won the game.

My own game is a bit similar in that it's easy to die early on but then, as you get stronger, you'll find youself one-hitting most enemies and the XP flowing in at a huge rate. Of course, in my game this is countered a bit by the exponential growth of skill costs but once you get to the point where a skill becomes impossibly expensive to afford the next level you probably don't need it any higher.

Just a few points for people giving up early on.