Again, there is no one rule here, as we all vary, but generally speaking, using our own thirst as a guide is pretty helpful. How much you need to drink will depend on weather, your physical mass, how much you are exerting yourself and so on. If you are thirsty, drink enough during your run to quench the thirst, but don’t feel you need to finish the bottle. Due to the body’s limitations on processing water in the body, it is actually far more effective to maintain hydrated through drinking smaller amounts throughout the day, rather than finishing your run, downing 4 pints of water, and thinking that will suffice. One of the reasons the queues for the portaloos at the start of any race are so long is that many people understand hydration is important for running performance, and thus drink a huge amount just before the race. Any water the body can’t process is eliminated, so you may just find you need the toilet more frequently to the detriment of your running performance. I won’t pretend nerves don’t play a factor here also of course!