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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.
A 5K runner might do this session for 30 minutes, a marathoner for upwards of 2 hours. You can of course make this much simpler and have some workouts with longer periods at one pace, Bakken says. You'll be surprised how much better you'll handle this type of work after two to three workouts..