The China Great Wall extends from the Hushan Great Wall in Liaoning to Jiayuguan Pass in Gansu. It takes about two years to finish walking all the great chinese wall. Here I think you mean the walk of some sections of the Great Wall, right? The developed sections such as Badaling and Mutianyu are safe to walk. But the wild sections such as Jiankou, Jinshanling and Simatai is not very safe, you should be very careful for every step. These sections are suitable for adventure. And there are vendors who will sell you water and food, but they are asking at pretty high prices. So I suggest you prepare enough food and water by yourself. Besides walking, you can take a cable car or slide to visit the wall, it will save your enery and the scenery from top is great!