Certainly, I admit there were roads for a carriage, but Mrs. Bennett knows that when Elizabeth talks of walking to Netherfield, she is describing a three-mile trek across less civilized non-pavements.

By the way, one does not catch cold from being in a rainstorm. Jane must have had a cold manifest at that time that was already forming on her prior to her ill-judged horseback ride. She was undoubtedly uncomfortable, soaked, and bedraggled when she arrived, but germs do not sicken someone just because they are wet, no matter what folks in Author Austen's time thought!