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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.
Finishing times for marathons (26.2 miles) range from a little over two hours for world-class, elite marathoners to 8+ hours for walkers. In terms of average marathon times, the median marathon finishing time in 2015 for men in U.S. marathons was 4:20:13 (9:55/mile pace), according to Running USA. The median finishing time for women was 4:45:30 (10:53/mile pace). Before running your marathon, it's definitely helpful to have an estimate of your marathon finishing time, so you know how to pace yourself properly.