I wanted to run as fast now (aged 47) as I did when I was 30. My previous PB was 2:58 so I chose this as my target time and, adding 1% to the distance for GPS error and mishaps, selected 6:45 as my target pace.
I ran it almost perfectly, but at the the end my Garmin said 26.7 miles and 3:00:18. Reviewing the data, the GPS seemed to lose its accuracy a bit in the Docklands. What is most annoying though is that I was not too tired after the race. I'm not too stiff today and know I could have run faster if I'd realised I needed to.
Still, age adjusted it's a 2:44, so a better performance, just not a better time.
The moral is: run to the organiser's miles, not to your own. It's their race you have to finish.