2. At that time of year the weather can be anything from nice and cold to stupidly warm. 2012 - Cool and windy. 2013 - Cool. 2014 - Hot, feeling warm in the start pens before you even started running warm. Apparently 2011 was a heatwave!.
Meh, 2011 was nothing
Well, it was too damned hot - wherever you were at about 11:30 am that's where the wheels fell off. If I recall only those looking for about 3hours escaped the worst of it.
2007 was awful though - ATM will tell you - it was over 30c and people panicked and took extra water at aid stations, meaning the water was pretty much not available until 10 miles or so for those running slower than 4 hour pace.
I think that re-assessing VDOT during a training programme is a major reason for tune up races. Many of us use a half 3-4 weeks before the target marathon as our goal time predictor but assessing an changing intensities in line with VDOT every few weeks throughout - or at the end of a mesocycle - is just as valid.
I think there's some things to watch for when inferring marathon/other race times from shorter races.
First up, you want the distance you're predicting from to be as close as possible to the target one as a 5k is very very different from a marathon in terms of what's required - realistically that probably means a half. Similarly it assumes specific quality training for the marathon. Lastly, any predictor assumes equal aptitude for different distances yet we're all genetically and psychologically better disposed to - one of us might be able to blast a fantastic mile time but it doesn't mean they'd be equally good at an ultra or vice versa.