As you've probably read in previous reviews, take toilet paper/wet wipes! Most toilets (often of the squatting variety) had no paper, including the bathrooms in accomdation. I took snacks, but actually it was unnecessary as we got plenty to eat at meals and Kevin bought us fruit to eat during the trip. Take a towel.
The walkng mostly involves steep ascents and descents (and a lot of stairs). Parts of it is challenging, but after a couple of days of walking you get used to it all. Often there were options for people cut the walk short should they feel like they have walked enough already. Also, should you for any reason not want to walk on any of the days, then this wasn't a problem either. Don't let the thought of not being fit enough put you off! There were plenty of breaks.
Coffee-lovers take instant coffee, there's always hot water available at mealtimes.
You will have to wear long trousers at least once because of long vegetation on the paths (including some stingy nettles). It's worth having a long sleeved top with you in your bag when you will be walking through the vegetation as some people were all scratched up. I benefited a lot from zip off trousers.
Take an alarm clock, there are plenty of early starts!
If you can't use chopsticks, then take a fork and spoon (often there were none available).
Don't worry about not fitting into the group. There were plenty of solo travellers in our group. These types of holidays are pretty self-selective, wanting to walk the Great Wall of China already gives you something in common with the others. Further, eating together, walking together and Firewater will help bond friendships quickly!