Which part of the Great Wall of China is the longest continuous segment that you can walk (and/or climb) on (that is accessible and still stands as a wall) without disobeying no go signs?
I have walked through Simatai and Badaling segments of Great Wall. Badaling segment is longer (around 7 miles). Simatai is twice shorter (3.5 miles) but way more strenuous, so if you're looking for more exercise and not merely longer walk, Simatai is more difficult. It is also much less crowded, and scenery is better; Badaling is over-restored in my opinion. However Badaling is way more accessible with public transport (and thus more crowded).
Note that with both of them you enter the wall at its one side, then walk to the other side, hit the dead end and then walk all the way back.You can only save a part of it by taking the cable car down, but you still need to get back to the station which is roughly to the middle of the wall. So the distance you walk doubles; keep that in mind when plan your walk.