More distance, obviously, but you also need to work on your nutrition intake. You can easily do a sprint wihtout taking any fluid or solids in. A half is different. Find foos that work for you and whatever combination of sports drinks that works for you over long periods of time, You are looking at somewhere between 5-8 hours of exertion for a half, so you need to have a plan in place for that length of time, and practice it prior to the event.