Apple does indeed offer a 2 meter USB cable on their site for $29.00. I personally know of 2 other people plus myself who use it and it works just fine. It's also okay to use the standard 1 meter cable with the adapter plugged into a regular AC extension cord. They'll both work equally as well, so your decision may be based on looks or convenience in your particular application. As for 3rd party USB cords, my personal preference is to stick with Apple or Apple certified. FWIW.
You can use an extension cord as long as you don't experience any problems or issues with it. I've never had a problem with it. Some people have had problems with their device not charging properly. Not sure why. May be cheap cords. ?