If you are having a rough day but had planned on running just focus on getting out there. If you aren't able to do your planned run then at least you know that you tried. Most of the time though it is just a matter of showing up.
I have had a few days already where I barely made it to the trail but once I got there I was so happy that I showed up. My last run I hadn't properly fueled for the day and it was a hot bright day. I only made it 2 out of the planned 4 miles but I was glad that I had at least tried.