I had a tough run just yesterday - it wasn't bad per se, I finished my workout a couple seconds off pace, it just felt hard. Taking today off since I think it's a sign that I need some extra rest - just need to survive my long run tomorrow and then I'm taking a cutback next week.
My most recent horrific awful runs were at the end of January. I went out for my midweek 8 miler from work on a Thursday, started feeling unexpected IT band-related knee twinges around mile 3. Stopped and stretched a bit a few times, made it to 4.8 miles before I had to give up and walk to the nearest bus stop - fortunately I had my student ID on me, otherwise I'd have been walking the 2+ miles back. I was dressed for a run, not for waiting in the cold, so by the time I got back to the lab I was so cold I was shaking on top of being mentally defeated. Gave my leg a few days off, did some stretching and foam rolling, all was fine while walking so I went out to run a few days later thinking it would be fine. At mile 2.7 the pain was so abrupt and awful that I couldn't move forward anymore. The shortest way home was still another 2 miles and it was 17 degrees out. I walked down the street sobbing for a minute, ended up alternating fast running (mid-low 7:00 pace) with walking recoveries to get home as quickly as possible without freezing since going fast for short bursts hurt way less than trying to move at my usual easy pace.
Got home and cried for a few minutes, got mad, called to set up an appointment with a PT to get myself fixed and back on track with HM training, and emailed my coach to let him know what was going on. Being proactive instead of denying that I had an injury or taking a wait-and-see approach definitely helped me feel less defeated by my stupid body.
I learned that a good PT is worth their weight in gold, and just keeping up with a few really simple strength exercises/activating the glutes is all I have to do to keep the IT bands happy. I love lifting heavy, but jumping back into that too quickly on top of my running schedule was more than my legs could handle this time around.