Heart rates for training runs and races.

Maximum heart rate (MHR) - Resting Heart Rate (RHR) = Working Heart Rate (WHR).

Training zones are normally 60-70%, 70-80%, 80-90%, 90-100%

WHR x 60% + RHR = BPM, your easy run shouldn't be less than 139bpm.
WHR x 70% + RHR = BPM, your easy run shouldn't exceed 152bpm.

I only use heart rates for training, not racing. Going by your 10miles in 92mins with average HR 152, that made that an easy run, i'd say you'll easilly beat 4 hours. When I went under 4 hrs my half time time was only 1:55, and for 3:41 my half was only 1:49.

If I was you, i'd carry on as you are, then race day i'd set off at 9 m/m (supposing the race is soon and your pb is somewhere close) no more, no less, then if at 23 miles it feels easy up the pace.

Your HR profile is fairly similar to mine.  My max HR is a bit higher, HR for a half tends to be around mid 170s and I do long runs around 140, easy runs perhaps a little lower.  I would guess you could comfortably run a marathon at an average HR around 160, but there are some provisos.  Firstly, I would expect HR during a race to be higher than in training for a given pace.  (Not exactly sure why, perhaps the effects of the taper plus race day adrenaline?), also different people may suffer from cardiac drift by a larger or smaller amount which will effect the overall average.  (You may want to start out at a lower HR and let it increase later in the race to maintain the same pace.)

So it's difficult to be prescriptive but I agree with lardarse that from your 10 mile run, you should be capable of running under 4 hours.  You've still got time to experiment before the race.  Why not try some "marathon pace" runs in training, using different approaches to pacing?  Try running according to min/mile pace first of all, then try running according to a target HR, and compare the two.

So I think I'm progressing well. Last Sunday I ran the flatish Perkins GER in 1:57. Averaging 9:00/mile but with the first 4 miles around 8:30/mile (whooops) before settling in properly. Average HR was 168 or 80% WHR.

My Friday night 'speeedwork' session has been 4 miles getting faster now at just about 8:00/mile.

This Sunday morning I ran 18miles in 3:19 which is 11:00/mile at average HR of 152 or 67% WHR. Plenty left in the tank at the end, but that seems a high WHR and I suspect it's due to the half marathon last week.

Two weeks to the marathon. Not exactly sure of my plan for the day, I suspect it will involve aimjing to start at 9:15/mile and getting carried away at 9:00/mile

Hi TimR. I prefer the simplicity of % of Maximum HR, rather than using Working HR. I've never heard a particularly good explanation as to why this additional complication of using two variables rather than one, is any benefit (but happy to be convinced if anyone wants to give it a try!)

Establishing MHR is no mean feat in itself. There are various ways. I did mine in a lab.  Eminent sports coaches like Jack Daniels have published performance tables which reverse engineer ideal HRs into running paces. I strongly suggest you have a look at a VDOT calculator to see what it says - all the theory is written up in the book Jack Daniel's Running formula which is a good read.

Anyway, I'll stick my neck out and offer these HR zones for different levels of training (the fitter and stronger you are as an athlete, the better your ability to healthily sustain work at the higher end of these zones):
Racing up to HMs (92-97% MHR)
Racing in marathons (86-90% MHR)
Intervals (93-95% MHR)
Threshold (tempo) runs (88-91% MHR)
Steady runs (81-87% MHR)
Easy runs (73-80% MHR)
Recovery runs (65-72% MHR)

Hope this is simple enough and helpful. Cheers.