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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.
On average though, a mile can be walked in about 15-20 minutes. Therefore walking a half-marathon will take an average of 3-4 hours depending on how fast you are walking per mile. This of course will depend on where you live and what your preferences are.