I was thinking the same thing. The Vikings would have been pretty stupid planning a plundering raid on 5,000 miles of open ocean.
Vikings (via Denmark) would definitely have known about the British Isles since their Angle and Saxon relatives had invaded that island hundreds of years earlier.
5000 miles? To the US maybe... To Northumberland, it’s a straight shot of about 200 miles dead West from Denmark.
From what I understand, the series takes place around 800 AD. This is about when raiders started hitting the Isles.
At the end of the day, it’s a fictional series with the trappings of historical research. Don’t take it as gospel.