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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.
If he is an average person in good shape and runs only a mile, he may finish in seven minutes. But if he is running a marathon, that speed would often be decreased to nine minutes per mile or more, because the runner must conserve energy to complete the 26-mile race.