Posted: 12/11/2015 at 17:47
Hi Marty, the idea is that on race day (having rested for up to 3 weeks, carboloaded, and even perhaps having a caffeine gel every half hour ...!...) you'll be able to perform a bit better than in the middle of training. Of course the whole point of training is to do something that you're not perhaps as good at as you'd like, to frighten the body into adapting to be better at it in case you decide to do it again!
So it is quite normal for the sort of performance you hope for to be a real effort in training. If it weren't, then arguably you could set yourself higher standards
I have a similar rule of thumb to HRR - if I can do 10 miles continuous at planned marathon pace ("PMP"); 20 mile long run at an average pace of PMP+10% (so 9:59 m/m if PMP is just over 9 m/m); and a 20 mile long run with the last 10 miles at close to PMP (if you squint a bit) then I have a chance of doing PMP for the whole 26.2 (Garmin will say 26.4) on the day. Of course you can roll those 3 into one run
You still need luck on the day, you know, no wind, no warmth, a stable tummy, good health, a happy mood and a clear head and no potholes