The 16 years were mostly peaceful and although Guo Jing has to remain alert but that doesn't mean he would be busy almost all day long without any free time. Besides, most of this 'training soldiers, modifying defences' thing are probably handled by generals assigned to those posts anyway and not by Guo Jing himself. Guo Jing doesn't go around managing everything. And therefore, Guo Jing will have plenty of time to practice and refine his skills as much as any other Greats. And there's this fact that one of his main enemies is a GREAT level fighter, how can he afford to neglect training, it will be foolish and suicidal if he does. Guo Jing does not all of a sudden start thinking 'Hey, I know that one of my biggest enemies Jinlun Guoshi is still alive and he's on par with me, therefore I will train less and let him kick my a$$ next time we meet'. And the reason that he did not significantly surpass the Greats, it is probably not because he hardly practiced, it's just that Jin Yong just wants all the Greats to be around equal regardless of anything else.