I note in Ch.32 when Darcy makes that comment to her about about fifty miles being an easy distance of family and friends. Darcy's mind was preoccupied. He certainly hadn't called at the parsonage to make small talk and be sociable.He is struggling with feelings for Lizzy and he's trying to sum up Lizzy's attachment to Hertfordshire rather than consider the distance for Mrs Collins... For the reason you suggested, namely better horses Darcy probably does consider fifty miles quite easy. He does not have to shift around a family by carriage and in P&P2 spends alot of time travelling on horseback.