Seriously, we tend to work ourselves into a tizzy over relatively minor stuff when we're not out riding our bicycles. If there's something you really need, or really, really, really want to add to the load, go ahead. I note you've not said what would add 13 pounds to your load, except books and extra water. Forget the textbooks, or see if you can get them on a light Kindle, notebook, or similar if you need to study for something like a licensing exam. Check the route ahead, and take extra water on the days when you might need it; across most of the east and midwest you'll have small towns or convenience stores spaced so you only need water for 2-3 hours of riding.
It's not really about how much you can carry -- unless you want to make the Adventure Cycling blog for "heaviest bike weighed this year!"