Well you should find out how far you can run comfortably, then slowly start running longer distances. I found that when I first started doing cardio I was downing water like nobodies business, so definitely try to keep hydrated. Don't drink too much water or too fast because it might cause cramping. If you've ever watched andre agassi play tennis he takes like a million tiny sips of water, I started doing the same and I rarely get cramps anymore.