I do a slight take on that which I found worked for me quite well during these pretty tiring times.
Usually my normal workout would be 30-45 minutes of a guided class (peloton, etc). But in days where I'm just not feeling it, I select a bite sized class (~10 minutes).
If after that short class I'm still feeling lethargic, then I call it a day, still being quite satisfied that I completed a "full" class.
But usually I find that after "pressing play" my energy levels are restored and I end up adding another 20 minutes class on top.
I feel this adds the benefit of knowing that there's a clear early "exit path" in which there is no act of quitting, since you do something till completion.