Training means, that you know when your body needs nutrition and liquids. Especially at longer workouts (longer than 1,5 hours) you should not forget nutrition and liquids. Also in competition it’s essential to feed your body with new energy at the right moment.
How much should I drink? This depends on many factors (intensity, temperature, etc.).
If you do triathlon, don’t forget that you should drink on your bike because it’s easier than during running. In general, you should drink as much as you lose.
If you have a digital scale, measure your weight before your workout and then shortly after your workout and then take the difference of these two values. You have to do that in various situations, so you get your liquids consumption. If the weather is hotter, you need more liquids – easy or? 🙂
Additionally you need nutrient and salt in your liquids. Pure water is good, mixed apple juice is great. I always take natural salt in my liquids with me.