Hello and welcome to the Great Wall Forum!
Thank you for your question. Your plan sounds very interesting! You are correct; the path is not very easy to find. There are several ways to get from the road through Gubeikou (the JingCheng Expressway) to the Great Wall. None of them are easy to find. Also, there is one military area where you must leave the wall, bypass the military area, and regain the wall on the other side.
You should be able to make it in one day; others have. Our member
Fauster did this trek in two days recently. I will be interested to find out if you have the time and energy to climb up the Simatai ridge on the East side of the Simatai reservoir once you reach the reservoir!
The path is pretty slippery and steep in some places, and especially in the Gubeikou Panlongshan area, where the wall is not in good condition, you will not likely be able to run the entire distance.
You can find complete GPS tracks of the route at
Gubeikou to Jinshanling and
Jinshanling to Simatai, where they can be downloaded in several formats.
Good luck to you! Please let us know about your results after you return. Have a great trip!