Critical speed from recent hard runs, or why there isn't one

Critical speed: the boundary between running hard that can be sustained for a long time and running hard that can't, worked out from at least three of the athlete's own hardest efforts of 3 to 15 minutes in the window, each from a different road or track run (not trail or treadmill; a critical speed test's two hard efforts count separately), grade-adjusted and normalised for altitude. A change under 5% between two fits is within what GPS alone can move it.