Dealing with exhaustion and stress

I called in sick today - a thing that I rarely do (I did it once last year, I think, but had not done it for years before then.)

I just felt that I needed to get over a bad case of sheer exhaustion. I have felt extremely tired overall, and quite depressed about the amount of work that I am facing. I also have two high school students visit days to deal with this week (one tomorrow, and one on Thursday), and I am playing a fairly important part in both. I need to be fairly well to give a decent presentation on both days.

I also need to catch up with the plain, routine grading that being overwhelmed with other tasks has prevented me from attending to.

I think that I badly need to get myself to the gym on a regular basis to rebuild my stamina. But I can't seem to figure out when I could possibly find the time to do this.