I'm not a pro but do ok for an old guy.
PR's
5K - 22:30
10K - 49:18 (hilly course still need to work on this time)
Half Marathon 1:42
Marathon - 4:28
I have run 4 marathons and about 5 half marathons. You didn't say if this was your first half or you had run others. If it is your first your goal should be to finish upright and alive with all of your limbs still attached! If you don't have a training plan Hal Higdon's have worked very well for me in the past.
http://www.halhigdon.com/training/51...aining-Program