When asked the question, “What tells your body that you are properly hydrated?” I would answer as follows:
Basically, your body is not very good at telling you that you are dehydrated. When you feel thirsty you have already been dehydrated for a while.
So you should drink sufficiently enough and sufficiently frequently each day. The ‘official’ figures that a non-exercising person needs 3 litres are wrong. From experience, for me and my exercise levels, I need about 2 litres of fluid intake per day that includes not just water but all other sources. Maybe it’s a little more than that but not 3 litres.
How can you tell you are NOT dehydrated? Well, essentially your urine is clear or faintly straw coloured. It gets darker if you are dehydrated or if your body is getting rid of some unusual stuff (in layman’s terms).
I am also sceptical about drinking too much water. Overhydration can cause leaching of salts from the body with dire physical consequences. But obviously, it is best to stay hydrated. As far as a race strategy goes I would point you to the last time you saw a race at the Olympics. Do you ever see anyone stopping for water or even drinking water other than in the Marathon? No, it slows you down too much. And those guys are really going for it physically, so you’ll probably be OK over 5k without extra water (well actually you DEFINITELY will be OK over 5k providing you start out even reasonably hydrated)
The simple way to tell how much water you need to drink for exercise is to weigh yourself naked before and after a typical workout. The difference in weight is the weight lost from sweat. 1kg of weight loss is one litre of water. Simple.