Water is very heavy and you try not to carry too much, but running out of water in the middle of nowhere is an experience you never want to repeat. It is hard to judge how much to take – it depends on the heat of the day, the terrain and whether you intend to get lost.
If you do run short, you can always ask at a house, or even use their garden tap if nobody is home. There are sometimes water troughs – lavoirs – along the way, which we have often drunk from with no ill effects.
Another sure source of water is graveyards, which always have a tap. And of course any bar will refill your bottles, usually with lovely iced water.