You can keep peanut butter in a pack for ... months! But you have to have a container that will not leak oil! You can keep dense, homemade bread for several weeks. It may get a bit dry, but it will be fine. Store bought fluff, does not do so well, but in a plastic bag, protected from damp, it should easily last a week. Of course, in both cases, this is not to say that it will be at its best, but when you are hungry it will definitely do.
Yes... How long depends on the type of bread, and your tolerance for staleness ;) Peanut butter doesn't need to be refrigerated so it'll keep just fine. If you're eating 2 peanut butter sandwiches each time, you can go through a loaf pretty quick. I don't think you'll have time to worry about mold or anything like that.