I don’t really have advice to help, but almost made a post about how hard it is to purposely slow down in training! After asking DopeyBadger for advice a page or two back, I followed his pacing suggestions this week. It’s much harder than I thought it’d be to slow down by a minute or so per mile! Even on my 13 miler yesterday when I increased my intervals from 45/30 to 45/45, I still ended up having to really pull back and walk more a couple of miles to hit my targets.
I wasn't aware you were a run/walker. The paces (more specifically the long run pace) was quoted as a continuous runner using Hal Higdon. But if you're run/walking with a current fitness of a 2:28 HM, then I'd defer to the Galloway Long Run recommendation of +2 minutes from Marathon Tempo.
So if your Marathon Tempo is an 11:48 min/mile, then your long run pace should be a 13:48 min/mile. If you choose to use a 45/30 ratio, then that would be roughly the following (assuming a 16 min/mile walk):
12:40 min/mile run for 45 seconds with a 16 min/mile walk for 30 seconds. As you can see, to no surprise to me, when you adjust the long run to the Galloway run/walk recommendation of +2 minutes, your running portion pace (12:40) lines up quite nicely with my continuous runner "long run pace" (12:39). Thus, it's in the easy zone physiologically speaking. If you were to stick to the 12:39 like I originally quoted you for a continuous runner, then your paces would look more like this:
Suddenly that easy pace (physiologically speaking) of 12:40 becomes an 11:05 min/mile. Now refer back to the pace chart and you see the dramatic difference. Now instead of an "easy" 45 sec run at 12:40, you're doing roughly 10k pace (11:05) for 45 seconds. I believe there's a big difference to how your body responds to these two different training paces. That's why if your run/walking you want to aim for the +2 minutes recommendation.
One thing to keep in mind. Galloway is currently recommending 30 seconds for the walking break. He claims according to his research (which I have yet to find publicly available) that 30 seconds of walking represents a diminishing returns point. So for 0-30 seconds of walking you reap certain benefits, but after 30 seconds the benefits of walking begin to diminish vs the run pace necessary to make up for it. It's my belief that Galloway has settled on 30 second walk to help force runners to slow down the running portion pace. Much more difficult to maintain a less useful physiological pace for longer when the max walking break is 30 seconds.
For instance, if I adjusted your 12:38 min/mile long run (for continuous runner) to a 45/45 instead of a 45/30, you'd get the following:
Suddenly, that run pace (10:28) is now up close to 5k physiological pacing. Now an even bigger difference between 5k pace vs easy pace.
It is a learning process on trying to run slower. It typically takes about 2-3 weeks for it to become normal for either continuous runners or run/walkers. It's not easy to run slow in the beginning, but it will become easier. Although, I will admit I know some runners who have tried to run slower and it's taken them months for it to become normal feeling. The adjustment is different for all kinds.
Does anyone have advice to offer as to how to scale down your pace for the next few long training runs? I felt fine at the end of the half yesterday, but I don't think I felt like I could run another 13 miles! I have been doing 4:1 run:walk (every 5 minutes I walk for a minute). Cut the ratio in half? Any other advice in transitioning to run/walking a full?
Your HM current fitness is a ~2:20.
This puts your estimated Marathon Tempo pace at 11:08. This means your Galloway run/walk long run pace should be 13:08 (again +2 min). If you were to use a 240 second run and 60 second walk, then the pace would be like the following (assuming 16 min/mile walk).
A 12:34 min/mile run for 4 minutes and a 16 min/mile walk for 60 seconds. As you can see from the pace chart above, the 12:34 min/mile falls easily in your "easy" pacing zone. No need to change the ratio if you currently like it. The only recommendation I'd make is consider changing the walk portion to 30 seconds as per Galloway's current recommendations. But, if it's working well for you, then there may be no need to adjust it.
Depending on the plan you're using you may mix in some other paces (like Marathon Tempo or 10k). At the end of the day, the marathon is a 99% aerobic event. Which means your performance on race day will be dictated by having enough training in the slower section of the pace spectrum which helps boost aerobic performance. I call the easy zone anything from an 11:55 min/mile (long run continuous running) and the Aerobic Threshold is the Marathon Tempo (11:08 min/mile). So the more training you can do at or slower than that (11:55 or slower), the better. I typically schedule my athletes around 80% easy (defined by long run pace or slower (11:55)) and 20% hard (anything faster than long run pace).
Like others have said, just remember that whatever training plan you're following you are only half way through it. Could you have run another 13 miles after yesterday? Probably not. But that's because you were running at HM Tempo and not M Tempo, and because if you could have run 26.2 miles yesterday then why waste your time continuing to train? The purpose of the training will be to have you prepared to run 26.2 in 9 weeks, not necessarily today. I got a similar message from one of my athletes worrying about running a marathon today (after running W&D 10k and HM the past two days and doing Dopey). My message was the same. I ran 15 miles yesterday and would be in no condition to race 26.2 miles today. Trust the training and keep putting in the work and the training plan will get you there.