First of all, you are a very fast runner and you should be proud of your speed. You say you could be about 12 minutes faster if you could maintain your 20m pace for a full 26.2 miles. Here are two recommendations:
1. Don't walk. Try to maintain a pace that you can sustain for the full 26.2 miles. I find personally that walking late in a marathon can help me finish a tough course, but the walking is not helpful as far as achieving a respectable pace.
2. Next season, enter two marathons, about a month apart. You may have to walk or slow down on the last 6.2 miles of the first marathon, but this "training" run should help prepare you for a great finishing time on your second marathon. In other words, your current performance seems to indicate that you could use at least one more long run prior to your goal marathons.
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