Because for the first time in, what, ever, I didn't feel like lying down for a 3-day nap after my speed workout yesterday. In fact, I felt downright peppy afterwards.
(Hmmm. Perhaps I wasn't doing the repeats fast enough.)
The stats: 5 X 4:00 at 5K pace, which is 7:30/mile. The rest intervals were 3:30 at 10:00/mile pace.
In the last repeat, I felt so good that I went a teensy bit faster, to 7:19/mile pace, for the last two minutes.
What helped was that I had a rest day on Tuesday. In which I did no exercise. Well, okay, I walked a few miles. But that's it.
Raising glass of energy Vitamin Water, in a toast: Here's to rest days!