Don ' t even think of starting on a hike that takes you more than a 2 km from home without a bottle of water along. You should have at least two litres of water with you and drink 100 ml – 200 ml every 30 to 45 minutes. Keep the water coming into your body even if you don ' t really feel very thirsty. If you are hiking, you are losing moisture and you need to replenish it.
By the end of a 4 hour hike, you should have drunk all two litres of water and you should be able to use the toilet. If you don ' t need to, then all that water came out as perspiration and you still need to drink more water to stay hydrated. After a hike, you should drink additional water until you need to use the toilet. This doesn’ t mean chug it down, but it means drink about 100 ml every 5 minutes or so.
You ' ll have air to breathe unless you ' re under water or in a cave-in. If you run out of food, you can struggle on for 250 km if needed. But, if you run out of water, you have only a day or so to figure out a solution.