The most common problem arises in those who sweat profusely, as they are at risk of both dehydration and sodium depletion. Many athletes don't drink enough on a day to day basis and often go into events already partially dehydrated. Fluid is absorbed faster if small amounts are drunk regularly. If already dehydrated, fluid will be absorbed slowly and may sit in your stomach or cause a stitch during running. Dehydration can also cause gastric upset, so runner's diarrhoea can be caused by lack of fluid as much as by the food you eat beforehand.
Dehydration can also affect mental functioning and co-ordination.
Start by drinking regularily throughout the day. Don't save up until training. Check the colour of your urine. During exercise fluid losses should be matched to sweat rate. Obviously, the more you sweat, the more you lose.