At first Elizabeth had not seriously thought of going to Hunsford, but now she decided to go - why?

Elizabeth had initially been worried about her sister's health. After seeing her, Elizabeth's spirits rose, leaving her excited by the thought of a northern tour. 

Every object in the next day’s journey was new and interesting to Elizabeth; and her spirits were in a state of enjoyment; for she had seen her sister looking so well as to banish all fear for her health, and the prospect of her northern tour was a constant source of delight.