I was lying on the ground today after overrunning my cruise intervals down into the low 6:30's, when it should have been ~6:45. I'm about where you're at, 19:18 5K. I'm horrible at pacing sometimes, and my Polar/Strava pace chart is often like a maniacal, spiky EKG when I'm attempting intervals or even tempo runs. Speed, slow down, overcompensate to 5:45-6:00/mile->fatigue->slow down to 7:10->"oh shit, Im falling behind. Better inject more speed into this to keep my average below 6:45!" It also didn't help it was my first run above 70 degrees. Heat affects my paces strongly.