It seems Darcy has shown up to start "practicing" being sociable after Lizzy's comment at the Rosings piano. He is surprised to find Lizzy alone, which in a way limits his ability to spread his conversation around the room. But I imagine he is already being tempted towards the proposal soon to come. His reference to the Collins's marriage is an indirect means of drawing Lizzy into a conversation about marriage in general. Indeed, she shuts down the conversation when it suddenly turns personal to her: "You cannot have been always at Longbourn." (Ch. 32)

But by then he has sounded Lizzy out on a woman's marrying at a distance from her family: "what is fifty miles of good road? ... anything beyond the very neighbourhood of Longbourn, I suppose, would appear far."

So Darcy may still be (or think he is) fighting against proposing, but in his conversation (and those subsequent unrelated questions on his inexplicable walks with Lizzy) he is sounding her out and trying to learn more about her.