Hi, Jinshanling to Simatai is a popular hiking route. It needs about four hours to finish the trek. It basically fulfills your requirements except there are some sections really dangerous. Many bricks are loose lack of reparation. Some watch towers are crumbling especially when it winds. You can do the walk from Jinshanling to Simatai in one day and get back from Simatai by hiring a car back to Beijing at Simatai Parking Lot. It's no problem. But if you do not like this unrepaired section, you can visit Badaling or Mutianyu, which are well repaired with no crumbling watch towers. Good trip!

I have just read your email and hope this is not too late. My wife and I (both in our late 40s) just did this very walk from Jinshanling to simatai < it was fantastic you have the wall to yourselves and will easily complete the walk in the allocated time of 4 and half hours.the cost was $26.00 and was great value for money. if you need any help, shoot me an email,,,,,ntlworld.com|wayne.tracey1
We are also from the U.k