When I could run a 2:45 marathon, I could run a 1:16 high half marathon. I could easily coast through 6:00 pace for at least 6-7 miles and at the time I think max mileage was 75-80 miles.
If you tell me that right now you can run a 1:23 half, that indicates low 2:50s potential, sub 2:50 if you absolutely crush the next 18 weeks. I think 2:45 will be a stretch.
What you need to focus on - which is what was described above - is to improve your lactate threshhold pace.
You should be able to tempo a 10 miler at 6:00 pace and come out of that feeling pretty solid. 5 milers - you should be able to run in about 28 flat - 5:36 pace.
Long term, you want to be able to do a 12 mile tempo, give or take, in the middle of a 20-22 miler at 6:15 pace (2:45 marathon goal pace), and not feel like you just crushed your legs the following morning.
Your results right now indicate you have some potential to hit all of this (I remember when my best was a 1:24 in the half, and I worked hard to get it down to 1:12) ... but you have some hard work ahead of you. Probably about 6-8 months if you really attack it.