Hi, Jim,
I walked the Camino de Madrid in spring, so water wasn't as much of an issue, but I did carry 3liters with me on the walk over the mountains to Segovia. We met several day hikers on that walk, and they showed us good drinking spots, though, so I didn't really need to have that much. The last couple of hours into Segovia is where you will need the water the most because it's wide open, treeless and sunny.
I had a day or two when I didn't find any bars or shops open along the way. From Segovia to Santa Maria el Real, for instance, is 32 kms, and it was a Sunday. There were bars in many of the places we walked through, but not one was open. I always take fruit, nuts, and yoghurt in my pack, since I eat my main meal when I arrive at my destination. That was almost never a problem. The only town I found with no restaurant or bar serving food was in Castromonte, but the 91 year old bar owner and his wife got some eggs from their chicken coop, sliced some chorizo, and added a salad and bread for me. Castromonte has no grocery either, but there is a traveling truck that comes through, and I was lucky enough to come upon it. The other places I stayed all had some kind of shop, but Sundays are hit or miss in terms of shop openings.
In general, I would say that you shouldn't worry about water too much, not like the Vdlp, because you will be going through many small towns were there are always a few people even if there are no bars or shops, and I think you could always ask them for a refill for your water bottle. I'd bring at least 1.5 L just to be safe.
Buen camino to you and your wife, I really enjoyed this Camino. Are you starting from the Plaza de Castilla? I assume you have Johnnie Walker's online guide, and I have a few updates that haven't yet been incorporated into the guide. I'd be happy to email them to you if you send me a PM. Laurie