From what I understand, there is a white distance meter at the top, though it is not clear. It is also a ridiculously long distance before you reach the end (I think). I never did reach the end.
I do agree with adding an incentive to make decisions of whether to not or to continue staying still. You are correct that you can hide behind an obstacle forever without being punished, though you technically will be punished - punished with wasting time. However, a timer would be great in making such decisions. Score could be calculated by time and distance.
I think difficulty can be increased with distance as well, as the game is too static. The UFO always so slowly turns red, and the game is too long that it gets boring without new elements. The UFO attack delay can be reduced, there could be more enemies, etc.