I've been training on 90/30 as well. Is it common to cut the run interval a bit, for the actual marathon? Does that help you get across the finish line?

I'll take any tips I can get for my first!
@DopeyBadger can weigh in with a more nuanced response, but to me the answer is....it all depends. My easy / long run training pace is a bit slower than my "A" race marathon pace, so if I want to PR the day I would increase the running interval a bit and keep the 30 second walk time. But....at my age and current fitness level marathons are not an "A" race and my goal is just to finish upright and enjoy the experience. Given that, the last couple of marathons I have decreased the run interval a bit from my training pace while keeping the 30 second walk time.
I'll use an example of a 2:30 HM/ 5:12 M runner with an 18 min/mile comfortable walking pace.
The calculator I designed is a mixture of Galloway with Daniels. It proposes two options: A - Slow run, long duration, and B - Fast run, short duration. The comfortable walking pace plays a role here as well. Not everyone has the same comfortable walking pace. I know some runners whose walking pace is around a 16 min/mile, and someone like me who is closer to 21-22 min/mile. If we were both to do our comfortable walking pace and the same interval duration, then I'd have to run far faster to make up for my slower walk. With that being said, my calculator suggests an Easy/LR pace of 90/30 using a 13:00/18:00 pace, and the M Tempo can either be 100/30 at 10:53/18:00 or 50/30 at 10:00/18:00. In option A, you're doing 10k/LT pace for 100 seconds and then walking for 30 seconds. In Option B, you're doing 3k pace for 50 seconds and then walking for 30 seconds. So the answer is, you can go either direction. Some runners are better at fast/short and others at slow/long.
For completeness, here is Galloway's recommendation from his website:
A 5:12 marathoner has a 13:55 min/mile Easy/LR pace, as my calculator does.
For a 13:55 min/mile, he suggests 30/30, 20/20 or 15/15. For the M Tempo, he suggests 60/30, 40/20, 30/15, 30/30, or 20/20. So from this, it would appear Galloway is more likely to suggest a longer running duration for M Tempo than his typical Easy/LR pace. For me, I tend to suggest a purely aerobic pace for the easy/LR run pace, and thus my run durations tend to be longer than his. Whereas, he schedules a faster/shorter pace/duration for the run segment on the easy/LR workouts.
Lastly, here was the pacer listing from the 2019 WDW Marathon:
For a 5:15 marathon, they used 60/30. So you can see how all of these things line up compared to each other.