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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.
Marathon runners can average 8.8mph for 26.2 miles. Best I can tell and would have to agree is the average human jogs at about 8mph, of course distance is the great variable. There is a range in between walking speed 4-5kmh/h and 100m record sprinting 37.58km/h (Usain Bolt 2009). Generally for longer distances average speeds are half those of professionals, so for a marathon 2 hours becomes 4 hours or 10.5km/h.