Workbench consists of six disks, not one (Workbench, Extras, Fonts, Storage, Locale and Install).
You are not allowed to download ADF files of Workbench if you don't own the original disks. That you formerly owned them, does not matter. You can find them in Google, though.
If you still have your Amiga and the Workbench disks, you can create ADF files from them and transfer them to the PC yourself.
However, if you seriously consider using AmigaOS on WinUAE, you should buy the AmigaForever package. Not only to get all the needed files preinstalled, but also to support the remaining Amiga community.
http://www.amigaforever.com/Bye,
Thomas