This is a bit of a stretch NoTr & Wrangler, but something to think about. Your descriptions made me think of my dog's illness. My now 14 1/3 yo standard (65lb) poodle began getting tired on long walks and once had trouble getting up a few years back. One day a year or so later, he just couldn't get up after lying down for a couple hours. No back legs at all. Later that day, he was fine. I noticed that his legs "bicycled" after jumping off a low wall. It was episodic. He'd be fine some days and have trouble at other times.
Our vet diagnosed him with canine lumbosacral disease. He's on a steroid and we take little walks a couple times a day to keep his leg muscles & nerves exercised, but it is degenerative and he's now barely able to walk, among other things. When he is walking, his back legs are all over the place at times and sometimes he gets them crossed up. He also rolls his back feet over / drags them at times, which I think is called a proprioceptive deficit, meaning basically, he can't feel his feet / tell where they are.
Basically, there is a slight mis-alignment in his spine where the last lumbar vertebra connects to the sacrum / pelvis that is pinching his spinal cord. It is visible on his X-rays. I don't think he can feel his legs most of the time.
My vet said if my dog was younger - i.e. 5 or 6, they could operate. My dog was ~12 when diagnosed.
I hope this was helpful.
