I say this at least three times a week to athletes.
For instance, a runner may be doing a 40 minute progression run. 20 minutes steady, 10 min faster, 5 min faster, 5 min fastest. They should end the workout saying “I could have run one more 5 minute segment.” If they can’t say that then they ran the progression run too hard.
“I could have run one more” holds true for longer repetitions as well. Let’s say you’re doing 4 x 2k. Those are long repetitions, but you should be able to say that you could have run a 5th one at the same pace, or even a touch faster, than the 4th one. Running a 5th one would have felt like a race and you would have had to go all out, but the point is that you could have done a 5th if you absolutely had to.
Obviously I could have titled this post “Don’t race your workouts” but instead just be able to say that you could have done one more at the end of every workout.