Meh. Even aside from what the rules say, it's better to do something new. Because it'll turn out bad.
See, writing that much that fast means that you don't have time to tweak, polish, or go over any details at all. It's purely an excercise in learning to get started and follow through- the hardest parts of writing. Quantity over quality.

So if you try to use characters and ideas you've already put a lot of time into, you'll just end up hating yourself for butchering them.