I did my first marathon after years of multiple halfs each year. And because I was doing multiple halfs each year, my base longest long runs were 10 miles for long periods of time. That base mileage gave me a LOT to work with going into a first marathon: experience with how my body felt with various distances/weather/fuel; a solid base of endurance and strength, which meant that big mileage build in marathon training took less of a toll; a good understanding of time commitments and how life outside of running would need to be managed.
Assuming your longest long runs have never been more than 5 miles yet, I’d strongly recommend starting a mileage build now. You can use a Galloway half plan, but the basic is: 2-3 short runs per week, and once you get to 8 or 9 miles, adding a mile to your long run every other week, and a cut-back to half that distance on the opposite week. So your weekly long runs might look like this:
Week 1 - 5 miles
Week 2 - 6 miles
Week 3 - 7 miles
Week 4 - 8 miles
Week 5 - 4miles
Week 6 - 9 miles
Week 7 - 4.5 miles
Week 8 - 10 miles
Week 9 - 5 miles
And then you can play with that a bit, keeping your longest runs in the 8-10 range by repeating the long-cut back-long of weeks 4-9 until you get to that point of the marathon plan. Or go all the way up to 13.1 and schedule a race to get experience with that in the process.
As I and a lot of others will attest, you do not need to hit 26.2 in training to have a good marathon race. So IMO a great thing about Galloway’s plans going so high in mileage is that it leaves room for life to happen. Can’t do that 18 miler in its scheduled week because of weather? No problem: reschedule it for what would have been 20 and go from there. I created my own plans, but I plan for one more really long run than I actually need, just to create some wiggle room.
One other thing I’ll comment on is pace. I am MUCH faster at shorter distances than I ever was at 26.2. I think the majority of us are. Don’t try to hang on to your short distance pace as you go up in mileage - it’s normal and expected that long distances will require a slower pace. You may like slowing down by simply running slower, or you may prefer shortening your run intervals and walking longer: this is the kind of thing you can experiment with while building that base.