I think that's very tight. Rush hour traffic into Paris from CDG can be agonizingly slow. Customs is usually pretty quick. The Montparnasse station is big and sometimes you have to walk a long way to your train. A noon to 1pm departure would be better.
Rabedelas Calzadas, Hontanas, Castrojeriz, Boadilla del Camino, Población de Campos, Carrión de los Condes, Cazadillada de la Cueza, Moritanos, Sahagún, El Burgo Ranero, Reliegos, Puente Villarente.
14 days would be a very comfortable journey. It would allow 2 days to cross the Pyrénées and a rest day in Pamplona and a couple of days in Logroño as well.
If you absolutely need private rooms I wouldn't take chances in May. Try looking at booking.com for towns along the way and reserve. Most are fully refundable.
Our advice is to take it slower, put in a few rest days, stop 2 or 3 times for food/rest breaks and keep the kms to between 15 and 20 per day. We finished the Camino in May, 2023 (77 and 81 years). It's doable but in a reasonable fashion. You and your sister would probably do it much faster. Be...
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