Some thoughts based on the above
Galloway's recommendations are definitely no more than 30s walk intervals. The greatest portion of your recovery all happens within the first 30s of it, so you are just slowing yourself down more when you walk > 30s. That being said, I will definitely end up walking > 30s if it's particularly hilly.
I have a hard time running much slower than 12:00 min pace when I'm actually running, too. I'm 6'0" tall with long legs... even when I try to shuffle at my slowest shuffle, I can't really go much slower than that, and it's actually harder for me to run that slow sometimes than it is to move forward faster. I feel like I am plodding along and landing with a lot of heavy downward force on my knees when I try to run that slow. While I certainly have had plenty of runs where my pace was 13:00 or more, the slowdowns all come from walking more than normal.
So running is pretty basic. Apply a stress and the body responds with an adaptation. Different stresses = different adaptations. The more variety of stresses applied the more different adaptations gained.
For continuous running, there are two basic variables:
1) Pace
2) Duration
You play with these two variables to elicit different responses. Speed work, HM paced work, M paced work, Long Run, Easy running, Recovery running, etc. all elicit different responses.
For run/walk, there are five basic variables:
1) Run pace
2) Run duration
3) Walk pace
4) Walk duration
5) Total duration
You play with these five variables to elicit different responses. But if you continuously stay with the same run/walk intervals then you are always applying the same stress and the same response. When you first start out using this system, you will see gains. Over time though, you will find that your improvement may stagnate. You'll likely increase your total duration training, but might not see much of any differences in improvements. I'd wonder if that might be because you might still be applying the same/or very similar stress, and thus since that adaptation has been maximized there isn't room for improvement until you try other stresses.
Now if you were a continuous runner and running a 3 hour HM, then I'd give you the following training paces:
With 80% of your training occurring at a 15:33 min/mile or slower. But that's an issue because you wouldn't be able to run slower than a 12:00 min/mile. So then we'd have to revisit the five variables to figure out the possible manipulations.
1) Run pace - Fixed at 12:00 min/mile (for your slowest easy runs)
2) Run duration - Manipulable
3) Walk pace - Fixed at 16:00 min/mile (or technically whatever your personal walking pace is. Mine is a 25:00 min/mile, but I'm a really slow walker)
4) Walk duration - Fixed at 30 sec (If we are to believe Galloway, then this is the magic cutoff. Physiologically it's possible that the most benefits are received here, but I would be interested in seeing the raw data that generated this statement. Does anyone know if he quotes a source in a book or scientific article?)
5) Total duration - Irrelevant for the purpose of attempting to figure out how to run slower. It's important for other reasons, just not this one.
So, we see that if we accept Galloway's 30 sec walk premise, and if 12:00 min/mile is the slowest you can run, then the only manipulable variable is the run duration. Here are four different paces slower than a 12:00 min/mile but the average pace per mile is manipulated using only the running duration as a variable. If you do 4 min:30 sec, you get a 12:21 min/mile. If you do 15 sec:30 sec, you get a 14:21 min/mile.
So, this could be one way to help to varying your pacing and getting more easy running in while not pushing too hard. The key to an easy day is that it should feel easy. You should never feel overly tired because of it. You should never be huffing and puffing during it. Your muscles may seem a tad sore in the beginning, but towards the end it should feel like you could run forever.
The key determinant in figuring how to proceed with pacing options is to figure out what deficit you have as a runner. Whatever your deficit, your best path to improvement is to work on that in your 20% hard workouts. Towards the end of the run when using run/walk, which goes first your breathing or the fatigue in your legs? Determining whether it is breathing or muscle fatigue let's us know where we should guide the training. You can also look at a race equivalency calculator. If you're 5K/10K PR is faster than the race equivalent HM PR, then you have the speed to run a HM faster what you lack is endurance (95% people fall here). If you're 5K/10K PR is slower than the race equivalent HM PR, then you have the endurance to run a HM faster what you lack is speed (2% people fall here). If you're 5K/10K PR is equal to the race equivalent HM PR, then you are doing great and should continue to balance both speed and endurance (3% people fall here).
@DopeyBadger Wow, that was an amazing breakdown! Thank you so much for sharing the science behind running. Can I ask for input too? My 5k PR is 34:39 and my 10k PR is 1:14:21. But my half marathon PR is 2:56:55. I use run/walk intervals because when I've tried to run a 5k the whole way through, my average time ends up being slower. My current intervals are 2 minutes running to 30 seconds walking. According to the pace calculator, I should be able to do a 2:45 half? I just haven't been able to do that. Now maybe it's just race conditions in that the half's have been hilly or windy/rainy and the 5k/10k was flat... My goal half in December is flat so I'm interested to see how that works out.
Thanks! Below would be the paces I would suggest you train at if you were to do continuous running.
Your 5K and 10K PR fall in line with each other. But your HM PR is a bit aways. Just from this I would suggest more endurance work. It would seem you have the necessary speed (VO2max) to run the desired 2:45, but that you currently lack the endurance to do it. The endurance comes from Lactate Threshold running and Running Economy. As a run/walker you are very likely already running Lactate Threshold paces (this would be roughly the pace you can race for an hour). So where you'll find the most gains is from working on Running Economy. Running Economy is the bodies ability to stave off fatigue (the ability to maintain the LT at a set pace for longer durations). This comes from more easy running, and more overall time spent running. So my suggestion would be to attempt to run 80% of your runs in the 13:38 or slower pace range. So in summary, your training paces are currently likely too hard. Completing more easy running may allow you to get faster. The two phrases I like are "Don't survive the training, thrive because of it" and "Save it for race day". Overall, the large majority of your training should feel rather easy. You should have very few days where your paces fade at the end of the training run. And additionally, you should find you are faster at the end than the beginning because you're so fresh. If that's not the case, then you're probably running too fast on a normal basis.
So let's say you ran three days a week:
Day 1 - Normal (Tempo) run/walk (2 min:30 sec) at whatever pace you plan to run the HM at. A 2:39:58 is a 12:13 min/mile. If doing 2 min: 30 sec, that would be a 11:32 min/mile run with a 16:00 min/mile walk.
Day 2 - Easy run/walk (30 sec easy: 30 sec walk). For the easy running, try going as slow as you comfortably can. Never huffing/puffing. Just nice and easy. These elicit "active recovery" and "cumulative fatigue".
Day 3 - Long Run (2 min easy: 30 sec). Try running for 2 min as slow as you comfortably can.
If life permits, you may find that this plan would be really easy and you could do more running per week. More running per week = more gains in running economy. This would be accomplished with more Day 2 running.
If you train for run/walk, and then attempt to continuous run a race (or alter your run/walk timing) you will likely find it to be less successful. Because the training elicits certain adaptations, you need to rely on those adaptations on race day. So when you train run/walk, but attempt to continuous run a race you aren't using all of the tools you've built up in training. You've trained your body to be efficient at the ability to clear fatigue with walking breaks. No walking breaks means no clearance of fatigue, which equals a slower time. If I train for continuous run, and then attempt to race using run/walk I would also be less successful because my tools from training are focused on continuous running. As I've said before a big key in determining which is right for you (run/walk or continuous run) is based on your physiological properties. But to successfully know whether you are better at run/walk or continuous run means you have to try it for a large majority of time (at least 4-6 weeks) to allow the body to adapt to it. I'm not planning on run/walking in Spring 2017 per se, but I am planning on incorporating more run/walk type running (speed intervals) to work on my current deficit (speed/lungs).