Now this is a great challenge! Take a watch or stopwatch to the track...or have someone there to time you. When you run those 2-200's at 80 and then 85% max effort/speed, record your times for future reference. The other part of this challenge is to not go so hard in the first rep that your second rep is slower. Really get a feel for the pacing. Sprinting while trying to hold your phone doesn't work, so that's why we're using watches.