Walking and hiking are often used interchangeably, but there is a difference. Walking us normally used to describe walking on a paved, relatively level surface such as a street or a sidewalk. Hiking is normally refers to walking off-road, on uneven terrain. Whoever devised these terms and definitions must not have had the Great Wall in mind, for it doesn't really fit either description. The Great Wall is paved in the east, but it's also hilly and uneven. In the west, it's more level, but it's not paved. Therefore, it's not wrong to use either term to apply to the Great Wall. To some people, hiking implies a more serious undertaking with commitment and involvement, where walking applies more to regular exercise mostly for its own sake.
Trekking usually refers to a long and adventurous walk, so if you're planning to walk for several days or more on the Great Wall then you can refer to your adventure as trekking. For shorter journeys, you can call it hiking or walking as you prefer.
The type of walk or hike you wish to do depends on how much time and effort you want to devote to the Great Wall. Many people start out with minimal plans and wish they had made more time available. Others start out with over-ambitious plans only to later decide that the effort or the danger is too great. For your first Great Wall walk, try to start in the middle; allow plenty of time, an absolute minimum of a full day, but preferably several days and several different locations. Don't commit yourself to a very long hike until you have tried it first and you know what to expect, or unless you have extensive hiking experience and feel that you can handle hiking the Great Wall.