You will always hear ppl say, "you need to stay hydrated". The issue is that you really won't understand the effects until you expose yourself to them. Yes, hydration is crucial for performance. It's like running your car with no coolant. It will run for a little while and then you will start to cause serious damage. Another issue is finding the right amount for YOU. You will see all sorts of estimations for this, but at the end of the day you are will have to determine the appropriate levels for yourself. Also, hydration is continuous. If you are feeling a little dehydrated before you run, its already to late.