Hello,Mr.bob
You can choose to go to Qianmen subway station, Fuchengmen subway station, or Chaoyangmen subway station. It takes about 40mins to 1 hour if you walk from Jingshan park to one of them. If you have to take subway line 2, take a taxi first. Or i suggest you walk to Xisi subway station (of line 1) and then transfer to line 2 as you can transfer to any lines with a 2RMB entrance fare if you do not exit.

there isn't a subway too near the Jingshan and you would have to walk a little bit but not very very far. If you are the type who enjoys walking, the stroll will be interesting because of the area is old and has many interesting things to see.

if you have time and energy, you can walk north up to gu lou(drum tower) and there you have the line 2 station called Gu Lou.

that will take you to lines 1 and 13 of the subway.