Yeah - something that's hinted at in the comics is that while Viltrumites can go a couple weeks without breathing, they do have to planet hop to a certain extent to replenish oxygen reserves. My thinking is they probably use some sort of anaerobic process to generate energy when there's no oxygen available for an extended period of time, leading to eventual cellular damage if they don't pause and 'recharge'.
Humans can do this for a short time via anaerobic glycolysis, Viltrumites have this turned by by 1000x given their physiology is 1000x more durable than ours.
Anyway, using a ship would avoid all this.
Nolan was able to reach a black hole with an active accretion disc within weeks of leaving Earth, and the nearest one of those is around 3,000 light years away, implying he can fly through space at 75,000x light speed, which is pretty incredible, but still means it'd take well over a year to cross the Milky Way. If their Empire is as vast as it hinted, that could be too slow to police it effectively.
Their ships may be faster yet.
)
