If I can add my thoughts on parkrun pacing insanity: when I ran my parkrun PB (which is not as fast as my 5k PB) I ran the first mile WAY too fast, partly because I accidentally found myself in first place for that mile and there was no-one to pace off. Also because I'm an idiot. Anyway, I knew as soon as I'd done it that I'd f***ed up, and so it proved. First mile in an idiotic 5:37, and then barely ducking under 6 minute miles for the rest, just slightly picking it up at the end. It was still a PB (average pace 5:51), but I finished knowing that more sensible early pacing would almost certainly have meant a slightly better AND more comfortable one.