Although there are some differences between the series and and the books, the concept of the Four Lands being in our future isn't one of them. The books are set several thousand years in the future, but I suspect the series isnt quite that far ahead in the future, which would explain why some buildings are still recognisable.
I believe the first mention of Shannara being set in the future was in the Druid of Shannara, part of the heritage of Shannara series of books. Notably the Eldwist is an ancient city turned to stone, whose buildings are described very much like our skyscapers (this was how I remember it, please someone correct me if I am wrong).
However in his books Terry Brooks has established an entire timeline extending from the late 20th century into the future history of the Shannara series of books.
Have a look at the Word and the Void series and the Genesis of Shannara series to see how our modern age turns into the world you see in the Shannara Chronicles.