Marathons are long and hard things to do.  They take time to prepare for, and frankly you aren’t going to do it overnight.  There are some people who choose to instead run a half marathon, which can be a good thing to do or it can be a building block in your process toward actually running a full marathon.  Either way, if you can pull off a full marathon, you are quite accomplished to say the least fitness wise anyway. 

Keep in mind at all times that most of them are for charity.  You aren’t competing, and frankly if the man that won ran it in 2 hours flat, that doesn’t mean that you have to run that fast.  Just finishing it is an amazing thing.  Now, preparing for a marathon in general will help you to run farther, longer, get cardio in general, and that’s a good thing.  You can increase your speed, length, and otherwise over time.  But just remember in so many ways that you have to do it at your own pace. 

Maybe you only increase your distance by 1/10 of a mile per week, maybe you get an extra ½ a mile each week.  It depends on the person, and you should never overdo it, because then you will just do more damage than good.  But all in all, it’s important to remember that you are doing a good thing for yourself any way you look at it.