So, its a little hotter than you wanted, a bit too humid, looks like it might rain, and maybe its an hour later than you wanted it to be. Its a Tuesday morning, you are supposed to do 5 miles. You don't feel great (but you aren't injured). What do you do?
A) run slowly and stop a bunch for water
B) Take the day off
C) Cross-train inside for a bit
D)Wait until that evening and reassess
The "better to undertrain" crowd would probably vote for B. Let me make the argument for A. I don't really think there is such a thing as junk miles with two exceptions, unless they lead to continuing really bad habits or likely to aggravate an existing injury they are just miles and you should do them. Neither rain nor snow nor gloom of night etc. Running when you are sub-par (but not if you're hurt) is too much a part of endurance running to skip in lieu of a sub-par run, which is better than no run at all. Nothing bad probably happens if you skip one short run, but keeping the habit going is even better, even if not optimal. No one can train purposefully and brilliantly day in day out but we can get out there, even if just for a slog. Plus tomorrow is only a day away.