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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.
The key to successful marathon training is consistently putting in enough weekly mileage to get your body accustomed to running for long periods of time. Newer runners may start with 15 to 20 miles per week total and gradually build to a peak week of 35 to 40 miles.