My partner and I would like to walk in the Jinshanling section of the great wall.

If you go on weekends and public holidays, you can take the holiday special line 8 at 08:00 from Dongzhimen to Jinshanling directly and return Beijing from Jinshanling at 16:00. The single journey will take about 2h.
Or you can take a train from Beijing North, Changping North or Qinghe Railway Station to Gubeikou, then take a taxi to Jinshanling and vice versa from Jinshanling to Beijing. The single journey takes about 2.5-3h.