1) 20 miles should be enough as the longest run. I generally max out at 22 in my trainings. Since you are running more slowly, you are running close to the time it will take you to finish - this helps your body to adjust
2) Don't think you can train and get a peak performance on your 1st marathon, there's too many things to learn about the training and the race to have an absolutely stellar outing as a novice (this is different if you are an elite runner)
3) To keep the schedule that you have written down with a big progression every single week is almost certainly going to injure you. To have those weeks consecutively and relatively safely, we would be talking about having a solid base of 100+ miles per week. I run 60-70 mpw and will have two peak weeks followed by a recovery week. Someone in the 40-50 mile range it should be peak (long run in the 20 mile range), fall back, peak - don't do 2 consecutive peak weeks.
Don't kill yourself before you get to the race. You will be able to finish with a few 20's under your belt. Progress to that level slowly (i.e. your schedule should not look like 12, 15, 18, 22 as your weekly long runs). Your body has to adapt to the demands of running that long. Max increase in weekly long runs should be 1-2 miles if you are running distances you have not recently run.
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