Just to add, my first marathon (aiming for sub 4) I ran halfway in 2.10 and got to 21 miles in 3.20, leaving me around 8min/miling to break 4. I felt great and upped the pace to finish in 3.58.
This year aiming for 3.40ish I went with the RW 3.30 pacers until about 20 miles when they sped off into the distance.Some how I managed to maintain something like a regular pace and finished in 3.31 although the last 10km was slower than the rest.
The most comforatble race was the former.
However despite years of practice even pace running is still difficult, the closest I came was during the Great South a few weeks back , my 5 out/5 back splits were within 5 or so seconds of each other.
I like Wurzels advice, which is more or less what i tried this year, Ok at 15, stick with it to 20/21, and if you feel good, try and up it , if not theres a long way to hang on for.....