I find in a 5K you can't make up the time lost from going too cautiously after the 1st mile really - so your first mile should be as fast as the rest if not faster. I'm all for going for it and trying to hang on
The last 5K I did I overdid it with a 6:18 first mile, then a 7:13, 7:35, would have been better a flat 7:00 min/mile pace (with a steep downhill in the 1st mile and back uphill in the last).
If I were you I'd try a 5:40-5:45 for the first 3K/4K then give it everything you have left for the rest as an interim measure at your next parkrun. If that works you can try running a 5:20-5:30 the week after. If your last K is faster you know you went out a bit too slow at the start so you can modify it next time.
Andi, I tend to have my 610 set to 0.25 mile splits rather than the default 1 mile - since I've changed it I can usually manage within 5 seconds per rmile over several miles as I get 3 chances to modify my pace after seeing it per mile. I'm useless at pacing myself without the watch!
If my legs recover I'm almost thinking of running the Buxton HM on Sunday - Eyam on Sunday was great (1:54:54), calves feeling destroyed now and still can't walk downstairs. Views were magnificent and well worth entering just for those. http://connect.garmin.com/activity/314879829if anyone is interested in seeing the profile correlating to speed (or lack of it!)