Question About Recent Quality Runs

I've noticed a phenomenon a few times recently after really great quality workouts / runs of late.

The hallmarks are: a hard, "quality" workout (e.g., long run with surges, several race pace or sub-race pace miles, lengthy tempo runs or speedwork like mile repeats) that requires a decent effort. This is never an "easy run" or "relaxed run" or "recovery run."

I'll get into a "cruise" mode where I am nailing every aspect of the run, feeling good, the effort is hard (but not maxed out). I'll always feel like I could keep going... almost indefinitely. I will have no real breaks in the run (no walk breaks anyway, there might be recovery jogs).

And then, the workout will be completed.

And then, I slow to a walk. And I suddenly went from a 3 on the tired / exhaustion scale (during the run) to about an 8 or a 9 the moment I stop. I'm just overwhelmed and think I couldn't run another step at that moment. Yet, seconds before.. I was cruising along just fine and could have kept going.

I also have the counter case. Fartlek runs with one minute surges are actually harder on me than longer surges. The one minute on, one minute off (say, 10 x 1 minute hard with one minute recovery) will fatigue me much more quickly during the run than just running, say, the same pace for 10 straight minutes. Or, at least I sense fatigue during the run much more quickly.

Feel free to write to me or comment on this issue if you've ever had it or can explain why it is happening. I assume it's a good thing, but I don't understand it.