Sorry to be gross but . . .

my dog's stools have been weird for the past several days. They're really long and kind of get stuck about half-way out of his butt.

Sorry, but I could NOT think of a more delicate way to put that.

He's 13. I haven't changed anything about his diet lately.

I think any change in bowel movements in an older dog that isn't precipitated by a change in diet or anything should probably be checked out by a vet.

I would personally wait another day or two. If the problem persists take him in.

You can try giving him some canned pumpkin (pure pumpkin, not pie filling).

That often helps "get things moving."