Start out easy man, it's important not to push yourself too far right of the bat or you'll just get discouraged and quit. Find an easy, comfortable pace and see how far you can go. A mile would be a good start. Another really good thing is interval training. Set a distance, say half a mile or a mile and get a stop watch. Sprint for 30 seconds (or whatever's comfortable), then walk for the same amount of time, then repeat until you're finished. It'll build up your endurance quick as hell.