The breath dials. They blow out air enough to keep it in the air on earth for a whole hour, that's a tremendous amount of force with which one can propel themselves in space where there are minimal drag forces, allowing for that force to make the ship go very very fast.
Considering how un-aerodynamic it is and how much metal is likely in it (almost an entire city of gold's worth), it would likely have around as much or more force than our spaceflights use to blast off into space. Also, considering that's a whole hour of constant use, it's quite possible it's more "fuel" that our spaceships have, thus allowing for the actual speed issue to be resolved.
As for the why he can't use lightning, lightning cannot travel through the vacuum of space due to a lack of any "medium" with which to flow through, like Enel does through the air, metal, or otherwise, since to varying degrees, they allow for a flow of electrons. However, it is difficult to expect electrons to flow through, what is to my knowledge, emptiness.
Assuming Enel isn't an exception in the no-air, no-food, etc. and that he intended to bring people with him, there must have either been a legitimate reason for not only him but also even him intending to bring Nami into space. Otherwise, it is just "plot armor" and they just don't want real-world logic to apply for some stuff.