I don't know if anyone is still reading this far down the board, but I just found another reference to an elastic walk in Middlemarch - "Dorothea walked across the street with her most elastic step and was quickly in her carriage again." In this case, she was shocked and upset, and trying to remove herself from a bad situation quickly. So maybe what they called elastic was what we would call "springy" - a step with a bounce in it, which may be happy, graceful, or urgent. If this is the case, I don't see anything inappropriate in General T's commenting on Catherine's elastic walk.