No, not at all.
You just have to create duplicate variables within your save game object and, within your game save function, assign the values in your game instance to the duplicate variables in the game save object.
Do the reverse within your load game function.
Since you’ve already got a game save object, and presumably game save and game load functions, all you have to do is create 5 variables in your game save object and set/get their values within the game save and game load methods.
Every time you add a variable you need to save for your game (character experience, current level, current stats, active quests, completed quests, whatever) you just want to add a duplicate variable to your game save so it can be saved to disc and loaded in the new session; and make sure you update those variables when you save/load the game.