You are probably not going to like what I have to say but from being able to run for 30 minutes, 2 1/2 months is quite short to train for a marathon . Assuming you run around 10 minute miles (picking it because it is a nice round number and about the speed I run) you are running 3 miles.
This training program http://www.halhigdon.com/marathon/Mar00novice.htm (which I used for my first marathon) assumes you could do a 'long' run of 6 miles and still takes 18 weeks to get you through to a marathon. You might be able to find a program that is less than 18 weeks but I would think that would start with a longer run than 3 miles. Some people consider Higdon's program to be close to the minumum you would want to do.
Regardless of what you think about his specific training program, Higdon's book http://www.halhigdon.com/books/bookorder.htm provides a good overall guide to marathon training



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