I can relate to this as I've had so many dogs with the same problem. No doubt you're describing the early signs of arthritis and there are a few things you can do. Entrophen, or Entrophen+ works well after a walk, I use it all the time. The idea of small booties for the back legs is a good one, that works as well especially on rough ground. Laying on a solid floor or concrete makes it worse, have somewhere soft to lay on. I use a wool blanket doubled over not too far away from a heat source. A massage now and then works wonders and so does swimming, best exercise you can give. Treated with care you can at least ease the discomfort and slow it down.