Long Run paces...how fast is too fast..

Agree with BernardDog's perspective.  I'm not accomplished runner (actually a non-athlete), but went sub-2 HM for 1st time last May.  Then posted another PR with ease  the next mo in much warmer more humid conditions.  From this slug's perspective going sub-2 really is about logging the miles (consistent 30+mpw) without killing your legs for the next next day/week's training plans.  If that means a few walk breaks to keep the pace under control that's fine (as rec by coaches Galloway, Higdon, & others). Any LR pace roughly in 10min +/- range is fine, as all the 'big name' plans seem to be about that range.  I usu find it easier to maintain 10min pace by running 9+min intervals with walk breaks vs trying to run steady 10min pace.  But find what works best for you.  Speedwork prob not necessary as you already have speed to go sub-2.  It's being on your feet for 2+hrs that will give you the endurance to maintain that speed for full HM. Only other suggestion might be for a little hill work (1-2x/wk) if your HM course is hilly. 

BTW- You mentioned being burned out at race day.  Did you taper for ~10-14 days pre-race?  Goal #1 for any race is getting to start line rested and UNinjured.

With a little luck I'm sure you will achieve your goal.

You should be fine at your current pace.  As the previous post said there is not much difference in the paces you indicate.  The other factor you mention is that your PR came at a time when you did not train much.  So if you factor in the additional fitness since the PR, your pace is probably right on. 

If you are really worried about it, run a 10K and put that into the calculator to update the paces to your new fitness level.

Good luck!

Are your logs accurate?  Based on the relatively few miles per month you are running, if so, I would focus on getting more miles in.  This might mean you have to slow it down, so that you can remain fresh and ready enough to run again the next day or run longer.  I am not trying to scold you, I think this is related.  Easier runs can mean more runs. 

I think using the conversational pace idea is a good one, it's my most effective tool to measure RPE.

By the way, I train my long runs around 11 min/miles and my pretty recent HM PR is 2:06.  So you can see it is possible to run a good race with slower runs.  I really never run a sub-10 mile in training unless I am doing speed work.  I am not sure there's a "too slow".

One concept I don't fully understand is the aerobic conditioning and how "too fast" can harm your progress.  I would worry about accidentally going over my lactate threshold. 

Caveat, I am not a good runner, but have had flashes promise in the last 10 years.  Usually I let something get in the way of taking it to the next level.

What I have experienced and witnessed with respect to the long run is simple.  Be aware of how much the long run contributes to your weekly mileage.  If 50%+ of your weekly mileage comes via your long run, its too much.  I used to despise long runs, because they hurt too much and took too long to recover from.  This negative experience week after week was ALWAYS a contributor to subsequent burn-out, a long break, and massive loss of fitness.  When I try to keep my long run at 25%-30% of weekly mileage and take the focus off of pace, I am never in too much pain, I stay consistent and I rarely feel the burnout.  I am not suggesting you can't cheat on this "rule" from time to time (I have had to in recent weeks), but consistently having the long run be a huge % of weekly mileage has been a disaster for me in the past and I have seen the same with many many training partners. 

With overall consistency and run frequency, you'll smash 2:06 this year!