Those people back then measured the distances neither in kilometers nor in miles but rather by how much time they needed to walk. For example, a village two days away meant a distance equivalent to walking for two days; ten days away meant a distance equivalent to walking for ten days... However in this verse the Quran specifies 1000 years of what they counted (not what they walked). Those people back then followed the lunar calendar and counted 12 lunar months each year. These months are related to the moon and not related to the sun. Hence in 1 day the angels will travel a distance of 1000 years of what they counted (the moon). Since this verse is referring to distance, then God is saying that angels travel in one day the same distance that the moon travels in 12000 lunar orbits. Outside gravitational fields this speed turned out to be the known speed of light.