I seem to be able to generate them from almost any activity. Certainly an unaccustomed run would do it.
And good for you to set a PR with already sore quads. I find the 1000 to be the most debilitating distance for making a hard effort. I bounce back more quickly from a 500, and I guess I can't push myself quite as hard when I have a whole 2000 to do.
maybe that means you are getting in 30-years-ago marathon shape!
....30-years-ago  marathon shape...
Ha ha! Good thought! However yesterday's run was only a bit more than 3 miles, not 26. But thanks for the encouragement.
And as far as my 4:07 PR today, seems to me that you did 1000m in something like 3:47 just last month. Very nice piece....but don't think you could have been too 'debilitated.' :-)