Many adult dogs lay around a lot, and sleep half the time. Una does this a bit excessively, however. She has a very calm temperament, which explains a lot, too. If her temperament was different, she probably would have developed other kinds of coping strategies to being left alone in her earlier life than those she is using now. She would have started maybe to pace round and round or left and right all over again (stereotypic behavior) to release her stress, or to destroy everything in her reach. I think she learned to do something similar than what most cats do in an environment that does not stimulate them enough: they start to sleep much more than usual.
Also, something I have noticed is Una licking, and also biting her paws. I think this might be her other stress-relieving strategy: repetitive action, licking and chewing her paws, releases endorphins just like in the other self-rewarding habits (or some other hormone related to pleasure, I don’t remember this accurately). The downside of this is that she injures the pads of her paws, even drawing a little blood sometimes. This must be one reason why it is hard for her to walk on gravel, with open sores on her paws! The cure for these kinds of self-rewarding habits is reducing the underlying reasons, in this case, giving her more activities so she lays around and sleeps after being very nicely tired, and not out of boredom. But even then the habit might continue, because it is self-rewarding. Time will tell.