We are staying in the Marriott City Wall - how long would it take to get out to Xiangshan Park?

For the hotel, do you refer to the one near the local railway station?
If so, from your hotel, it takes around two hours to two hours and a half reach the park by local public transportation.
From your hotel, you can walk to reach Railway Station or Jianguomen Station to take the local subway line 2 to reach Xizhimen Station. Get out of the station, you may find bus 634 to reach its terminal at east gate of the park.
The direct taxi costs about 80 to 90RMB.