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Assuming you can run 2 miles now at a nice easy pace (conversational), within 20 weeks you should be able to train up to being able to finish marathon distance at that nice easy pace (between 10 and 12 minute miles) or approximately 4 to 5 hours.
In marathon training it has been found that 3 runs of 18 - 22 miles over the 8 weeks prior to the marathon are an important predictor of completing the marathon. The long run is also an important element for middle distance runners.