Running

As posted above start slow. An extra 30s per km for the first 3/4 km's is'nt going to kill your time.

Both half and full marathon runners started at the same time on my last full marathon. I placed myself with some half runners and felt OK, if a little pushed, at their pace. I thought I was going to beat my PB, untill my body colapsed at 23 miles. Could have been for any number of reasons but I feel it was a case of too fast too soon.

Use this calculator to use your half time to calculate a full time.
http://www.liebreich.com/LDC/HTML/Various/RunCalculator.html

Your 20miles in 2:46, gives a marathon time of 3:44. But you cant really compare a racing time with a training time. Your half marathon time will be a more accurate measure. FWIW 3:30 is my (slightly ambitous) target time, and I doubt I could call a 2:46 - 20mile run 'comfortable'.

Once you have your target use the 'mile split times' to keep you on target during the race.

I might be exagerating to call it comfortable! more that i felt i could have gone if necessary and better than the 3 previous 20 milers I've done. Someone i know is running in the same race and he's about 9 min faster over 20 miles than me, i don't want to run with him if I'm going to run out of juice before the end.