Gavin,
Feel almost able to help here!
I ran 3.31 at FLM this year (and hoping to do better next year!) , this was based off of a 1.37 halfM in March and an unofficial (ie run in the gym as opposed to race) 10k of 43mins.5k time similarly is 20mins, but the gulf between 5ks and marathons is so great as to be irrelevant.
The halfM time *2 plus 10-15 minutes is about right, it seems to have worked for me over the years!