when do I get faster?

Given your HM time, your long runs seem on the slow side of "normal".  But how much do you run?  How often?  How long is your long run relative to the rest of your running?  How much harder running do you do?  How do you arrive at the 9-9:30 pace today? 

Just spitballing...  Do you recheck your LT?  You should do the test a couple times a year.  Your zones may have changed a little, or they could have been a little off in the first place.

I'd recommend to do the field LT test per this article (http://www.beginnertriathlete.com/cms/article-detail.asp?articleid=633) and use the BT LT HR zone calculator to see how the zones line up.  IMHO you need to reset your RPE/pace based on something better than max HR derived zone references.  Not sure how you determined your max HR for running but FWIW I've "maxed" out at 189 in a 5k and have an LT of 176 which puts my LT based Z2 at 150-160.

I can really relate to this question.  Like you I choose to do a lot of LHR running (Maffetone approach).  My max HR is 193, so very similar, so that gave me a LSD HR target of 143 a few years ago, and I've stuck with that..  I recently did a threshold test that puts my z2 top end at 150, so can push it harder if I want now but have used the 143 as a benchmark.  I started out doing this for marathons and it took me quite a few months to see the pace start to creep up at the target HR, but it did.  Slowly. 

My experience:  What I didn't realize was that my aerobic capacity had developed deeper than was demonstrated by these runs.  Again like you, my marathon times were around the 4 hour mark and based on my training I set my marathon paces at or near that.  Then, during the Eugene marathon I had a buddy I was going to be running with and we were going to shoot for a 3:50.  He got in GI trouble early on so our pace was not on target.  At about mile 8 or 10 he told me to go on without him as he could see it was going to be a bad day for him, so I mentally calculated (what I thought it would take) to get me in at about a 3:50 and started running to that.  I found that I could maintain that fairly easily so pushed a little harder.  Ended up running a 7 minute negative split on the back half of the course and ended up with a 3:42 and an average pace of 8:29 at an average HR of 155.

The takeaway for me using LHR as a training tool is that it can make you better than the training results demonstrate, because of the constraints you put on yourself using LHR.  It looks like you have demonstrated a similar deeper aerobic capacity with your HM of 1:42.  Using the MacMillan calculator, that time puts you capable of a Marathon time of 3:34 - so I think you are better than you realize.  Your last question of long run pace is a good one, and I think I was guilty of talking myself into the same thing - letting the LHR target dominate.  I did start doing some runs at a target HR of 153 and found I could get down to an 8:30 or so for distances of 10 miles or so, and started doing those runs about once every 2 weeks to give myself some confidence.  Over a few months the I saw my overall pace improve from the 9:00ish I'd be stuck at down to an 8:30 overall - at the target 143.   Sounds like it may be time for you to mix it up a bit pace-wise to see if that can break the cycle you are experiencing. 

I'm very skeptical of using Max HR to set zones.  Generally, people don't accurately determine it (it's VERY hard to get to your max in any kind of test), so you estimate it, and it's typically too low.  If that's the case, you may have been running in Z1 (based on if you determined your LT and set your zones with that).  Z1 running is not bad, but not as effective as Z2.  Also the reason you could maintain a higher HR comfortably (zones too low).

I did the Maffatone method for a while and the 180 - age (plus/minus adjustments) to set your AT/AeT threshold also seemed to be a little low for me.  In fact, if I had to do my training now using that, my max HR to stay aerobic would be 135.  That's REALLY low and just the easiest jog.  My Z2 based on LT for running is 153-163 and I can keep that going all day but it never feels "easy", just a comfortable effort.

Have you tried running a HM on just RPE?

I suspect that if you put away the monitor for a session and just went for it, you actually would be faster.

Ties with most of what was said above - your zones are likely too low (max HR is pretty much set, but the highest HR you can hold for a given period of time can increase - and you're prolly more fit than the zones you're using and might be holding yourself back).

That, and run more often.

Matt

And, as some have mentioned, don't forget the run "lots" part.

I keep fiddling with my 'zones' because every time I read another article, the percentages are a bit different.  in the end, it probably doesn't matter between a couple of beats here or there.  Here's how Joe Friel (a trusted source) sets up running zones (cycling zones are slightly different):

Zone 1 Less than 85% of LTHR     <_____

Zone 2 85% to 89% of LTHR  _____ - _____

Zone 3 90% to 94% of LTHR  _____ - _____

Zone 4 95% to 99% of LTHR  _____ - _____

Zone 5a 100% to 102% of LTHR     _____ - _____

Zone 5b 103% to 106% of LTHR     _____ - _____

Zone 5c More than 106% of LTHR    >_____

 So- for your LT of 159, it would come out:

Zone 1    103 to 135

Zone 2    135 to 142

Zone 3    142 to 149

Zone 4    149 to 157

Zone 5A   157 to 162

Zone 5B   162 to 169

Zone 5C   169 to 172

The importance of the above statement cannot be emphasized enough.  If we're talking about longer endurance events, HM and above and Oly tri distance and above, the amount of 'speed' work is surprisingly small.  5 to 15% of your total mileage (depending).  If you're running less than 40mpw, it might be even smaller than that.  As the old adage goes; speed work is icing on the cake.  first- you need to make cake.