I know it was dangerous to travel at night and probably not very comfortable either. I was only considering that Lady Catherine might have been told by her lady in waiting or the driver to rest a while and since she has not been used to submit to any personīs whims, as she tells Lizzy, she would insist that she has no intentions to follow the advice of anyone (so beneath her). It would have taken a lot more than 10 mph to travel under those conditions making the progress slow and she would not arrive that much earlier than she would have if she rested in London. The sensible thing would be to take a break. :)