Dear Wah:
i think i have your experience years ago in my first marathon
my first 30k 3:00:00
last 12k 1:50:00
i think it is 不夠長課
i train more long run in this two years, and in this year of my 4th marathon, i have my PB 3:56:58
half: 1:52:43
30k: 2:42
33K: 3:00:00
last 9k 0:57:00 ( actually i still drop a lot in my last 9km)
Total agree with shinji, as I can run sub 1:45 in half marathon, but I will exhaust in the last 21k, so control the speed with practice of more long run are also important in facing full marathon