why is "we run DURING the long month of July" not a good translation? in this scenario, both "in' and "during" serve to explain when the action will take place (from 1st to 31st of July)
It probably relates more to the amount of daylight, in the Netherlands in July the sun rises around 5.30 and sets around 22.00 with long twilight periods even shortening the night further. I read recently that there are a few weeks in Dutch summer when it doesn't get fully dark because there still is astronomical twilight (when the sun is less than 18 degrees below the horizon), more on twilight, see wikipedia.
Maybe it's meant the same way as you say "It was a long day" "Het was een lange dag", the day wasn't longer than usual, it just felt that way because it was a difficult or very busy day.