The most common question I get from people back home is: What's China like? My website: http://www.owendennis.com China blog: http://www.owendennis.com/blo...
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.
As the other answers, the distance of Boston marathon is 26.2 miles or 42.195 kilometers, just like all other marathon. But you need run really fast to be qualified to take part in Boston marathon, that will take a long time, that would be years long.