Think you're ready for that first marathon? Here's my advice for what it's worth. Run shorter distances first. Start with a 5K. Then do a 10K. After that, you will want to do a couple of 10-milers. Finally, a half marathon is advisable. After that you can reassess, knowing that you will have to run twice that distance in a full marathon.
This may take 6 months to a year. Personally, I did 8 half marathons (out of the 45 I've done) before I attempted my first full marathon in Cincinnati. This helped me figure out a goal pace for the marathon. Now that I've completed 20 marathons -- Cincy, Baltimore, Raleigh, Richmond, St. George (UT), Phoenix, Dallas, Fort Worth, DC, Allen (TX), Chicago, etc. -- I'm used to running 50K ultras. Keep in mind that training is more than just running. Strength training is important also. Not only does strength training offer protection for muscles and joints from the impact of long-distance running, but it can help you improve your running economy as well.
Please, please, please: identify what your goals are for your first marathon. If your goal is just to finish and you could care less about your time, that's great. If you want to shoot for a BQ (Boston Qualifying time), that's another story. Always be clear on your true "why?"
Hope this helps!