At first it sounds impossible, but it's not. People do it all over the world for days and even weeks on end. I remember vividly the second week living with my host family in Nicaragua. We had a big orange carton called a "pichinga" which we filled up with water from the well at the bottom of our garden and purified with a chlorine tablet. Occasionally we would have running water from a tap to be purified.
Why on earth would you want to cash in your entire pension pot at age 55 when all the statistics point to you living well into your 80s? For many people, the answer is that you would be crazy to do so - you should leave the pot alone to let it grow and be thankful you will have a pension to fall back on when you reach 65.