Regardless of which marathon you choose, you need WAY more than 6 weeks of training to be ready. It sounds like you would really prefer London. I'm not sure how likely/unlikely getting into London is, but it wouldn't hurt you to wait an extra year even at your age. You will still be improving year by year for the next 5+ years because you are so new, so if 2013 doesn't happen for you, 2014 is fine too. Just work your distances up to the 20-30 mile/week range and let your body get used to that. Once you are stable there, you will be ready for more advanced marathon training.