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.
The first and perhaps most obvious reason you might not want to run a marathon is a lack of training or experienced running background. The marathon is an arduous event and requires a dedicated training block of at least two months for serious runners and four months or more for newer runners.