Timed 10km run - slight improvement needed

This morning I completed a timed 10km run along Brighton seafront.

After a brief warm up I set off towards Hove Lagoon and back. The wind was light so had little effect to the overall time. Just after 9km I ran out of seafront a bit and had to include a couple of ramps and turns in the last 1km, which probably added a few seconds but generally it was as good as it gets - decent weather and a relatively flat course.

My total time was 45:56, average HR was 150bpm (max: 159bpm) and pace 4:36mins per km. I pressed the lap button after 5km and recorded the following splits:

L1 22:50, average HR 148bpm and pace 4:34
L2 23:06, average HR 153bpm and pace 4:37

I need to get somewhere close to 43:00 to give myself the best opportunity of PB in the marathon in April so some improvement is needed. After the session I felt OK and recovered quickly, thinking I could push harder on the next attempt.

Runner's World have a fun tool on their website - the Race Time Predictor: http://www.runnersworld.co.uk/general/rws-race-time-predictor/1681.html

Plugging in my recent 5km timed run of 21:27 I obtained the following predictions:

10km - 0:44:43 (suggesting today's time was on the slow side)
1/2 Marathon - 1:38:34

If I use today's 10km timed run of 45:56 I obtained the following predictions:

1/2 Marathon - 1:41:14

My PB for Barns Green was 1:40:58 so better than my run this morning, however I would have beat my 2010 and 2011 times.

In summary, some improvement is required, although I doubt I will make any further progress between now and the 30th September when Barns Green is scheduled. My focus will be on my 'A race' and beating my PB for the Brighton marathon in April.