You will want your urine color to be light, as in pale lemonade. If it looks like apple juice then you need more water.

Normally an endurance athlete will do a better job of keeping their electrolyte balances closer and their hydration up to par if they drink a sports drink containing carbohydrates, sodium, chloride, magnesium and potassium. This is especially true for a salty sweater.

Drink 500 ml two to three hours before you race or run for a long time, during the long term event drink between 150-350ml every fifteen to twenty minutes and after ward replace every pound of body weight lost with at least 450-675ml of fluid.

These drinks are tolerated easier if they are cold and with a flavor you enjoy. You will have to experiment as to how much and how often you need to drink. It is best to set up a schedule so you don't end up along the road in a severe dehydrated state.

By the time you are thirsty, you are already starting to get behind the eight ball.

You can go overboard with excess water (Hyponatremia is a metabolic condition in which there is not enough sodium (salt) in the body fluids outside the cells. http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001431/)
and end up with severe problems.