I hate to disagree with mspath, but I would carry 150-200 euros with me. I have found the maximum withdrawal in Spain for me is 300 euros. With that amount of cash, another town with an ATM is always within reach.
I walk with my wife, so there is always someone to watch our stash of cash, credit cards, and passports when one of us showers. So I am not so worried about theft.
Another tip mentioned earlier on this forum is to make withdrawals at a bank that is open...just in case the ATM machine eats your card. (This happened to me once, but the bank was open and the manager retrieved it.)
Buen Camino. And welcome to the throwback cash economy.
Tom