I created an ISO disc of Mint Qiana Cinnamon. It will create and run virtual machines no problem. But when I try to boot from the disc, I choose to boot Linux Mint, the disc spins for a few seconds, then stops, and nothing happens.

I have a 64 bit processor, and am using a 64 bit version of Linux.

I'm not sure what I'm doing wrong. I know the disc works, because I've run the virtual machines. Is there anything I should be doing differently to boot from the disc than for running virtual machines?

Good question.

Many ways to go here. One might be to boot the VM from the disc in question from the drive in the host unless you have booted the VM from the disc. (It is possible that you meant that you booted from iso image just to be sure)

If you say that you already have some version of linux running as the main OS and you simply can't get this disc to boot then might consider trying other disc's and also 32 bit OS.

What are specs of system?
What OS is on it now?

Just because a VM can boot from a disc doesn't fully prove the disc is good nor was the download correct and tested. Be sure to double check the disc and or iso image.