Your current program and your proposed plan for Kiawah are excellent.
If you can run 20 miles, you can finish 26.2 miles. You are probably asking whether you will be running or walking during the last 10k. With more experience about pacing, hydration, clothing, and other factors, you will be able to run at a steady pace the entire 26.2 miles. You just may not be able to do this on your first marathon.
Some of the running sites recommend that for mileage like 30 mpw that about half the total be put into the long run. For you, that would be a 15 mile long run and three other runs of 5 miles each. You are close to this at 12 miles for your long run.
Personally, I don't think short runs of 3 miles will help you much, unless the alternative is no run. The important variables for a successful marathon seem to be the distance of the long run and the total weekly mileage. Perhaps as you continue your training you can make a 10k (6.2 miles) your minimum run.
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