I don’t think I’m getting sick. I can’t be sick now; I’m off to Jamaica on Thursday.
But I had no energy today. But if I think about it, there is a good reason.
I did a nice reasonable run yesterday. 23 km. Short enough that it doesn’t dominate your day and just leaves you feeling refreshed.
That is all true if you don’t go for another 7 km run after lunch because a cute girl wanted to.
A 30 km run will drain you and mean you have to eat much more than you think. If you don’t account for that, your body will run out of energy and you will be lethargic the next day.

Guess what happened.