Gonna go slightly faster on the conversion. 1:18-low = 2:43, 1:18-high = 2:45. I think sub-2:50 makes sense as a good time goal. I wouldn't start at 2:43-2:45 pace for your first one.
As far as advice goes, mileage is key for the marathon. I like Pfitz. You need to train for the distance. Mid-week threshold workouts with good volume. Mid-week medium long run. Weekend long run with tempos @ GMP sequenced throughout your build.
As far as race day goes, take the first 10k ~5-10s/mile slower than goal pace. Take your first mile 15s slower than goal pace. No excuse for ruining your day early. Don't shrug off nutrition as knowing how to fuel up when a half is the longest distance you've covered. Practice with fluids/gels in training. The wall is avoidable is you pace & fuel right. It can also to be hard to eat/drink the later into the race you get.
Last bit of advice is to make your final pacing plans based off of race day weather. If a perfect debut for you is 2:49 but it's going to be over 60 on race day and/or windy and/or humid then make an adjustment. A perfect 2:49 day might be a 2:53 day. But if you got out at 2:49 pace it might mean not breaking 3 hours. It's easy to fall way off in a marathon compared to the shorter distances.
Anyways, good luck in your debut. Your mile/5k give you sub-2:35 potential in the future. You should knock out a BQ &, if you gain some of that speed back, you could run something much faster within a few years.