Darkness is localized. Hence he cannot travel across the country continuously at his current speed as in crossing outside of the field of darkness he would be unable to retain current speeds. His ability is also only retained at night, which is defined by the presence of natural sunlight. Hence while he could travel across the country using his speed, he can use another power far more easily to appear appropriately (his water travel abilities). And of course that is the point of the debate isn't it? You have science and I have magic. attempting to explain magic with science is tedious at best. As even you have yet to provide the formula which sustains life, (I may do so actually with having a Purge scientist, already working up on the concepts, I'd reckon it would be a throwback to modernized alchemy which how Crowley refined a sample for Bedivere. A similar process could be used to create the unknown formula. Although explaining the science of it's mechanics is tedious at best.)

As for the localization Currently it is over Kilo Point, and removal of his presence from Kilo Point, and hence Excalibur, would slowly return the area to its natural order of light. Doing the same to an entire country would take far longer, and cause much greater concern. Hence please do not find the set up in creating a battlefield of darkness (especially at night) so unreasonable to do in 2-3 posts. I mean your purge members managed to do far more in 2-3 posts with your machines in setting up the battlefield after all.

Anyways, we are in limbo.