Beware the Wind Chill

Just got back in from 7.2K. The training schedule called for 11.25K but it just didn't happen this morning. I hate to not follow the plan to a 'T' but it happens sometimes.
I felt a true sense of fatigue, pre-run, not just a mind game thing, but a physical fatigue. I had a feeling the running wasn't going to go well.
Possible contributing factors:
-didn't eat enough the last couple of days, ate on the run, and not enough protein
- did not get enough sleep last night
- the past three weeks I have really been pushing it with working extra shifts, catching up with paperwork on my day off, running 5x week, and all the Christmas preps
Weather:

 Notice the -20 windchill factor - That's -4F and love that sunset time, not! Bring on the Vit.D
So, the run was a mess of walking and running - hadn't had to do that in quite some time - had to keep running though because if I walked, my body didn't stay as warm. Damp + Wind = Hypothermia, it can happen fast, so kept up as best I could with the running. My legs just felt like they couldn't move. Even walking felt like an effort. Not sure what was going on there.
So:
How do you tell the difference between true physical fatigue, and your mind just playing games with you?
If it is fatigue, do you just push through on what the training schedule calls for that day, even if you have to crawl?
Do you make it a rest day instead?
Last Friday, I rocked the route at a 10k distance, shouldn't have been hard to add an extra 1.25k today.
Should I take tomorrow off - it's supposed to be an EZ 40min. and have Sat. Sun rest day and start fresh Monday?
Disappointed, but hey, that's the way it rolls sometimes- I figure what I did today will still help me toward running a good race on New Year's Eve. I hope I don't crash and burn like I did today. And I hope there's no damned wind!