Will drinking lots of water in the day(s) prior to a long trek mean I will need less water on the trek?
Your body is not having the capacity of storing large quantities of liquid. The fact is that the available solution in the body is part of blood plasma. Water cannot be stored like fat or carbohydrates in the form of energy. Water is not apart of the body’s storage mechanism. This is the very reason due to which you may be starved to death due to thirst very much beforehand you would be starved to death due to scarcity of food.
The body has some limits in optimizing water retention. After reaching that maximum hydration point, it will automatically throw out excessive water. In short, you urinate. This mechanism is controlled by the kidneys. Thus, technically there is no storage happening.
This can react adversely and put pressure on the kidneys and can lead to much lower levels of salts in the blood. So, you are not at all advised to drink lots of water. This can lead you to lose water through many processes such as urine, moisture, sweating.