Everybody makes out that the underground system in Paris involves stepping off one train, walking about five steps, and getting on another. Having lived in Paris, this is absolutely not the case.
The distance you have to travel (ie walk underground) in big stations like Chatelet-Les Halles, Montparnesse, Etoile is the equivalent of walking from Stephen's Green to College Green. It includes staircases etc and with luggage etc can be an absolute nightmare. I won't even mention the Tube, because it's worse again.