A marathon is 26 miles, so you have 21 more to go. I think you should start off at a very small amount increasing a half mile each day for five days, and then cut back a mile for a day and then increase it again. Eventually you will get up to 26 miles, but then before running a marathon you should train at a much smaller amount of miles. You don't want to actually run 26 miles more than maybe one other time other than before your actual marathon run.

         It will probably take from 6 months to a year to train for a marathon. There's a lot more to it than that though. You'll have to fun six times a week on a cycle of miles. week one, you should run 1, 2, 3, 4, 5, 6 miles, then week two 2, 3, 4, 5, 6, 7 and so on, for 6 weeks. Restart the process, hopefully running much faster miles than you ever have before. After that, you can begin funning much more steady quantities of miles each week. But you shouldn't train past 18 miles. Every marathoner I've ever spoken with says to run the last six miles of the marathon on adrenaline. Good luck. And make sure you eat lots of carbs and protein.

         yeah i would also say that its a long continuous process for training for a marathon if you really looking forward to complete it and may be winning the same. As it takes all your strength and stamina and they cant be build in a fortnight or for that matter in 10 fortnights. But if you are looking at it casually then i think training for 3 months regularly would be fine for one to compete it.

         Well it all depends on many different factors. How fit you are, how motivated you are, the equipment you have available, etc. If you are already in shape and run already you could train in a couple months. If you are starting from not a lot of running background I'd give yourself at least a year. The best advice is to find a trainer for a month who will get you on track and lay out a plan that best suits you. People tend to rush these things and end up getting hurt before the race.