Since it's better to learn from other people's mistakes than to repeat them, I asked people what they would do differently if they could, or when they do, walk the Great Wall again in the future. These were some of the most common answers:
I would allocate more time. Many people regretted having to leave the Great Wall before they were ready. This is especially true when people join tour groups for day trips to the Great Wall. These tour groups invariably don't allow sufficient time, a problem exacerbated by side trips to the Ming Tombs, shopping stops, leaving too late in the morning, and returning too early in the evening. Many people mention longingly eyeing the wall as it journeys into the distance and wishing they could follow it at least for a while, but not having enough time or worrying about missing the departure time. A trip to China is a major endeavor for most people, and the Great Wall is one of China's most impressive attractions. Don't short-change yourself. Plan ahead to allow yourself plenty of time at the Great Wall. Too much is really better than too little.
I would take more (better) photos. This common complaint usually involves a delay. Only upon return from their trip do many people realize that their photos are inadequate. And at the Great Wall, with its rare combination of highly impressive subject matter situated in an area of beautiful scenery, this is a common theme. People remember certain scenes or features, and wish they had representative photos. Having a camera that's handy and easy to use, and remembering to take photos regularly, will help you to avoid this mistake.
I would bring (more) water and/or food. Walking the Great Wall is an exertion beyond what many people are used to, and it takes time. Failure to carry dufficient food and water is a common and even dangerous error, but one that's easily avoided by a little bit of advance planning.
I would wear warmer clothes. Even when it's comfortable in the city, once you get out of town temperatures can be much cooler. And up onto mountain ridges, it can be quite windy as well, making it feel much colder. It's also common to encounter fog, mist, or rain at the Great Wall when the weather looks clear in town. If you aren't dressed adequately, enjoyment of walking the Great Wall can be greatly diminished. Avoid this mistake by being aware of the potential weather and dressing accordingly. It's easy to remove a layer of clothes, but if you don't have an extra layer, you can be very uncomfortable indeed. Be prepared.
I would be stronger. This is another mistake that results from underestimating the effort required to walk the Great Wall. It's the going up and down on steps and slopes that surprises many people that believe they are in good enough shape to walk as far as they wish on the Great Wall. And the best way to condition yourself for this is by walking up and down steps, or exercising on a stepper, for long periods of time. Note the stepper doesn't prepare you for the downhill steps which many people find a greater challenge than the uphill steps. If you plan to walk for a long time on the Great Wall in the mountains, be aware of this requirement and train for it to maximize your enjoyment.
I would use (more) sun protection. Unlike hiking in the woods, walking on the Great Wall exposes you to the sun almost constantly. Many people get sunburned on the Great Wall. Sunscreen and protective clothing such as a hat and a long-sleeve shirt are the answer.