They're basically a pile of mush by the end of their run, about to keel over and die. They don't have any fat and the muscles have been mostly used for energy as well. It gets significantly fishier smelling too, but like, not in a good way.

Some people like it. My dogs used to like to yank our shoulders out of the sockets on walks, trying to go roll around in the ones that flopped on to shore to die. That's not fun to scrub out of an excitable labrador.