Simply import the root folder of your hierarchy. Do that multiple times if you have more than one root. This will import everything in there. You will be able to navigate that old structure because Lightroom supports navigation by folders.
You may have been confused by some answers because in Lightroom there is a distinction between folders and collections but you can navigate both just as easily from the left pane in the Library view. The top structure represents folders, the bottom collections.
Collections are more flexible than folders because they do represent a disk directory and images can belong to multiple collections. To preserve your folder hierarchy, you do not need to use collections, simply let Lightroom import folders as folders.
That is exactly as I work. Things get organized by folders and imported in-place. From the Import Dialog make sure to choose the Add operation (it says Add Photos To Catalog Without Moving Them under that option when selected).
Then you can also use collections to organize things in orthogonal ways at the same time. So you can have images you've taken over several years in the same place (which would be in separate directories in your folders) to show up in one collection.