You can try using TestDisk to recover the partition, but unfourtunately, using DD is about the easiest way to destroy a disk (thats why its called DD) irrevocably, since it overwrites existing data.
you can find TestDisk, and PhotoRec on a number of system rescue disks. I've used this one before: http://www.ubuntu-rescue-remix.org/
TestDisk recovers partitions all in one peice, so hopefully it will be able to find and recover the partition for you.
PhotoRec is what you use when TestDisk fails. it searches the harddisk for unfragmented binary files with known headers and footers, to recover known file types like .mp3, .jpg etc. since your drive was only partially overwritten (I assume your ISO is nowhere near 500GB) you should be able to recover everything that wasn't overwritten as long as its a type Photorec can recognize.
Heres some great general info on Data recovery on linux: https://help.ubuntu.com/community/DataRecovery