I took part in a study a few years ago, where we had heartrate and body temperature data recorded after parkrun (we were described as untrained, recreational, middle aged runners, or something like that!) Anyway, it supposedly showed that maintaining core body temperature was a limiting factor for most of us even in relatively mild conditions.
One of the supposed reasons why the world record keeps getting broken at Berlin marathon rather than elsewhere is not that the course is especially flat or free of corners, but that it's typically run at a time of year when it's much cooler than other marathon majors - London, Chicago, NY etc. I've seen some graphs which show that parkrun PBs for people who've done a lot of parkruns mostly happen in spring & autumn in the UK.
PizzaSteve - we do something similar here - 10k run starting and ending at a pub every Weds - different pub every week. Good bit slower though, we run at the pace of the slowest, which usually means 10-10.5 minute miles and some of the ladies take a 7 or 8k option.