Moderation is a key skill. Slow down to just a bit less than your comfortable pace from earlier, and give me 30 strokes. Again, make sure you really feel all of them.
We aren't done yet, though. Go back to your comfortable pace, and give yourself 35 strokes like that for me. I hope that was enough for you to recover a bit of stamina.
Now, speed up again. Not too fast. 35 strokes. Just like that.