You have an airship that cruises at about 80 kph, give or take. Flying from Europe to the New York area will take about 3 days and change in still air. Prevailing winds, not well known at the time, would almost certainly be headwinds on the trip to the US so the flight would likely take longer. The result of this, given imperfect navigation primarily using dead reconning would result in an unknown time of arrival and the point of arrival off the (likely) Canadian coast would then require flying down it to the target (great circle route).
Given the low speed of the airship, it's likely to be sighted by numerous ships and coastal traffic, along with likely some observers ashore. Binoculars and telescopes aren't exactly rare items at the time so it's likely that the airship is flying in daylight at least in part of that. If you look at a MAP, even the distance from Halifax Canada to New York City is about 600 miles or about 12 hours flying time.
If the flight took place in summer--for better weather--NYC has somewhere between 13 and 15 hours of daylight meaning there's a damn good chance the attack is made in daytime. So, word is going to spread of an airship headed down the East Coast towards New York.
Even just flying across parts of Rhode Island, Massachusetts, and down Long Island would take several hours. The likelihood of a response by the USAAC is pretty high. The USAAC would have pilots, like instructors, who have lots of flying time and are skilled even if their combat time is low or nil. An airship isn't exactly a difficult target to shoot up.
Then there's weather. The Germans have ZERO idea what the weather will be at the target end of things. They could arrive and find a weather front with rain, the remnants of a tropical storm moving up the East Coast, or perfect flying weather. They may have to dodge a storm in transit too throwing off the flight schedule.
