Last June I registered for the Inaugural National Women's Half Marathon, even though I was still learning how to manage hallux rigidus and didn't know if I could actually run a mid-distance race again.
It was low-risk because they were running a special registration promotion - some slots for $1, some for $5, some for $10... When registration opened, I went right for a $10 slot figuring most people would try for the lowest ones first.
That strategy worked, and I registered for $10. I figured I could always transfer to the 8k, or even blow it off completely without losing much.