The problem with using inertia, is that you are assuming that you will NEVER encounter anything in space along your travels.
So let's say you're pulling a rock in space. Albert is right in that once you push or pull that rock (i.e. get it moving) it will continue on its way forever without needing any more pushing or pulling.
UNLESS.
UNLESS.....your rock becomes subject to the gravitational pull of, say, a planet (asteroids, for example). Then your rock will crash into that planet. Then you need guidance to NOT crash.
As to the speed - the Andromeda galaxy is like 2.5 million light years away. That's serious pre-planning, lol. 'Oh, I will need a star in 2.5 million years - better order it now!'
And that assumes light speed travel.