Note that scenic = Expensive!
The cable car over Mont Blanc from Chamonix, France to Courmayeur, Italy will cost at least 100 Euros. I can't think of a more spectacular boarder crossing so it is worth doing once in a lifetime.
Currently it is closed due to COVID.
Training is not always fun, but it does provide big advantages on the Camino:
1) Able to handle the Pyrenees mountains on Day 1
2) Less injuries, less time spent mending injuries
3) Able to focus mind on the Camino rather than the body aches
4) Able to take longer, alternate routes that are...
Train ticket in Europe are increasing being priced similar to plane tickets, based on supply and demand. Prices can change in a fraction of a second! This is especially true for high speed trains.
There is no magic in Sarria that will enable you to suddenly walk 100K! Walk at home in you local neighborhood park to determine how much you can comfortably walk. How may miles can you walk in a weekend? It's a great way to train your body and test out all your equipment. Don't wait until...
I've been to several events organized by the local chapter of the American Pilgrims on the Camino which have included a priest to bless our packs and poles. My local parish priest has happy to bless me with a rarely used pilgrimage prayer.
I suggest walking with the family at a local park and gradually work up to 25Km. This will give you a good idea if all members can walk the distance. Don't wait until Spain to start walking and endangering yourself and your family.
My understanding is that from Nov 1 - Mar 31, you can be fined for trespassing up to 12,000 Euros if found on the Napoleon route. I haven't hear of anyone actually be fined, but sooner or later someone will be fully prosecuted.
I've seen reports that a stolen US passport is worth about $2000 on the black market, which makes it your most valuable possession. Protect it, or you will have to spend a day or two of your valuable time at a US embassy waiting for a replacement.
There's always the story of a 2-pack a day smoker who lived into their ninety's!
Just because one person survived the the camino without training and an overweight pack does not make it a general rule. Maybe it was pure luck.
I'd rather not gamble with my health.
Note that Rail Europe is also the source in the US to purchase Eurail and country-specific train passes which can only be purchases in the US (and not in Europe), due to special pricing.
Eurail train passes would only of value if you plan to travel a lot around Europe before/after the Camino...