You should drink before you’re thirsty is myth
For most people, thirst is a sufficient indicator of hydration status. It’s a very reliable instinct that has managed to keep humans alive for a very long time. Thirst begins when the concentration of blood, an accurate indicator of our state of hydration, has risen by around 2%. Dehydration begins at about a 5% increase, so thirst signals a good time to drink and it comes soon enough to avoid any problems.
Water is not the only way
There really is no actual science behind the 2 liters or 8×8-oz glasses of water. It’s an arbitrary amount that is rarely going to be just right for you. Trusting your thirst is a better idea simply because water is not the only thing that hydrates you. Most foods contain liquids, especially things like fruits, vegetables, and soups can significantly increase your hydration status without any actual drinking. Contrary to popular belief, coffee and tea count too, their diuretic effects are too weak to decrease hydration.
Exercise is an exception
There are couple of exceptions where relying on you thirst might not be enough. A very intense physical activity is a good example. Imagine a long climb on a hot day. Your hydration status will deteriorate so rapidly that you have to make a conscious effort to drink more. Otherwise your performance will suffer quickly. If you want to keep performing at a high level in those conditions, you should replenish carbohydrates and electrolytes too. Especially sodium should be included in fluids consumed during exercise lasting for more than 2 hours.
The bottom line is you can trust your thirst to stay hydrated. When it’s not hot, you’re not exercising, and your foods are full of water, there’s no need to force yourself to drink the entire recommended amount. At the same time, when you go for a ride, it’s better to drink regularly rather than waiting to get thirsty.