Hi Rain Dog, sorry to hear that.
I had quite bad whiplash last summer when a white van drove into the back of me at traffic lights one Monday morning. I didn't run all week as I was too stiff and sore but had a 10k race on the Sunday that I didn't want to miss. I just ran it rather than raced it, wasn't too bad by then (I'd been on paracetamol and ibuprofen all week) Wouldn't have been able to race it though.
I eased my way back into training, as it was June I was post-spring marathon anyway.
I would really recommend you get physio as soon as you can. I had to wait 4 months for mine through the insurance thanks to a useless solicitor and developed a back problem that then developed into a bad injury halfway round New York marathon. I should have just gone through the NHS instead of waiting.
When do you need to make a decision by? If you could see how you feel even next week you might have come on a bit. An ultra is a whole different ball game to a 10k though!
Hope you're feeling better soon.