I'm sure your sensible enough to have plenty of waterproof and warm clothing so that shouldn't be an issue .. Pack your spare clothes individually in zip lock bags they are better than a rucksack liner because if that leaks all your clothes are wet, when your kit is individually packed in water tight bags it will always stay dry, for basic rations take anything that can come to life by adding water, hot or cold but also take packets of dried fruit and mixed nuts Incase water is scarce or you can't light a fire ... Take a fire Steele and cotton wool .. Again keep in a sealed bag to keep it dry