Based on a 10 miler at a little better than a 9 minute mile (I did exactly 9 minute mile), you get the following race equivalency profile (assuming these are recent times and this is a measure of current fitness):
View attachment 260163
Which puts your estimated HM at a 1:59:52.
The way I look at it is run/walk is simply continuous running but with speed intervals and rest breaks. So, the 60/30 run/walk can be at a variety of paces to garner the same average pace. But the ability to maintain that over a longer duration can be widely different based on the pace of the run and the pace of the walk.
For instance, this is a 60/30 with an average of a 10:20 min/mile:
View attachment 260164
This is a 8:47 min/mile run (or cover 0.113 miles when running) and a 16 min/mile walk (or cover 0.03 miles when walking)
This is also a 60/30 with an average of 10:20 min/mile:
View attachment 260165
This is a 8:01 min/mile run (or cover 0.124 miles when running) and a 24:30 min/mile walk (or cover 0.02 miles when walking)
The difference is the 8:01 vs the 8:47 running portion. For a person of your estimated race equivalency profile, that's roughly 2 mile pace vs 10k pace. Big difference between trying to run each of those for 60/30 splits over the course of a HM or M. So, it might be helpful to look at the pace when running to help determine whether it's a sustainable pace. I've worked with a few people who were doing 60/30 or 30/30 intervals but with the running portion at race equivalency 800m or 1 mile. That's an intense workout no matter who you are. I've found when working with these people that slowing down the run pace of the run/walk has allowed them to perform better.
A big thing to keep in mind is the Galloway LR should be at HM Tempo (or M Tempo dependent on the runDisney plan) + 2 minutes (at least how I understand his instructions). Which means someone with your race equivalency profile should be spending a significant portion of training at about a 11:34 min/mile pace or slower (M Tempo of 9:34 + 2 minutes). I believe he also recommends maintaining the same run/walk timing as you intend on using on race day. So if the timing stays the same, and the walking pace is consistent, then this means you have to slow down the running pace. Like this:
View attachment 260166
Now instead of a 8:47 min/mile, now you'd be doing a 10:10 min/mile. Not surprisingly to me, this puts the run portion at my scheduled "long run" pace. This means the grand majority of this run will stay nearly pure aerobic (or very easy).
This is just my best guess as to how to work through the math. Hope this helps!