Well, I had one of those days today. Included walking to the other side of town and back twice, lots of work, no time to myself, no time to run ... but around 7pm I set out to run, forcing myself. I was so tired I just wanted to collapse. But I managed the two ten-minute intervals at my normal speed. My legs were made of lead and my chest hurt and all I could think about was stopping, but I made it! I can't say I felt better afterwards, but I was relieved when it stopped.
It felt important to me to prove I could still do it even when very tired. Hopefully I'll be feeling better by Wednesday and so the next run will be easier.