inertial nav is not nearly accurate enough on its own. It needs regular updates, either from gps or dme.
Typical drift (even acceptable according to the manuals) is 2 + 2H in NM. In which H stands for hours since last allignment.
So this gives you a possible fault in your position of 4 NM after 1 hour flying. (assuming there are no updates available)
Don`t get me wrong, glass cockpits are great to work with, but use caution. Not everything you see is real.