The flu/virus/cold that's been going around my school (15% absent yesterday? yikes!) caught me Monday afternoon and did not want to let go. I went to school Tuesday but could barely speak; Wednesday I laid on the couch all day, napping. Today I was so excited to be able to get out of bed and shower without being out of breath. I returned to school, but my voice stayed away; maybe it'll come back tomorrow.
Instead of running 18 miles tomorrow, I will try to run 8. If I can do that without collapsing in Country Park, I'll try for 18 on Sunday.
I know that there's a lesson here, and I think I even know what it is: you can plan all you want, but life will sometimes force you to change course. I haven't ever had a running injury (perhaps because I don't run all that much?), but this little bout of nastiness teaches the same lesson. Sticking to the plan while ignoring the current situation would be stupid and counterproductive.