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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.
Elite runners will typically finish a full marathon in about 2:05:00-2:30:00'ish or so, so if you can keep up with them I give you lots of respect. Their averaging almost 4:00 miles the entire way. Most competitive marathons will give you about 12-14 hours to finish, but it varies from competition to competition.