I sat in bed last night trying to figure out how fast I could go on the treadmill. 6.2mph = 30 minutes, so if I do 2 minutes at 8.4 and 1 minute at 4.2 I'll get in far below my current PB. And if I run out of steam I can coast at 6.2 (I run at 6.2 for 5k fairly regularly) to stay in the running. So I tried it this evening, and ran out of steam at 20 minutes, so completed the run at 6.3 to make certain I didn't put all that effort in for nothing. I got in at 27:56. Now I know it's on the treadmill and not scientific etc, but I was astonished. That's 80 seconds off my previous time.
I always thought that constant pace was the way to get PBs, but is this an alternative, or was it just a really good day, or am I just lazy when doing parkrun and not trying hard enough?