You would use less fuel per increment, but you would cover a correspondingly shorter distance, so the maximum range of the ship will remain unchanged.
To use your example, you would travel at half the speed and consume half the fuel, but would cover half the distance. The end result would be that you simply doubled the time for your ship to make the journey, consuming the same total fuel.
The actual benefit of reducing your ship's speed is that you reduce the thermal signature of your ship, rendering it harder to detect by thermal scanners.
