I don't know which route has less mileage along the highway, but the VDLP doesn't have too much. There's some in the beginning from Sevilla to Santiponce, but that's a pretty short stretch, and in most other cases if you're walking along a blacktop road, it isn't well-traveled at all. The only stretch I really disliked was the walk from Xunquierra to Ourense. This is largely alongside paved roads, and it gets busier and busier the closer you get to Ourense. But it's one day's walk out of 6 or 7 weeks, so it's not a big deal. And there's an unofficial route around the Ourense section.
Re: water, while you DO have to be careful to carry lots of water, especially if you're walking when it's hot, I found there to be plenty of places to either purchase water or use fountains most of the time. There are definitely some stages where you're out of luck, as you don't pass through any towns the entire day and so have to carry quite a bit of water. And there are lots of towns where you'll find a fountain, but you're not sure if the water's potable or not and you can't always find people to ask. But I drink a ton of water and have never had an issue. There always seemed to be a fountain or store or nice person who refilled my mochila in their kitchen. (But maybe that's the magic of the Camino at work!)
Melanie
