Consider the marathon once you've been running about a year, three to five days a week, averaging 20 to 25 miles a week. Until then, enjoy and learn from your experiences with shorter-distance races. The more you know about racing those, the more prepared you'll be when you try the marathon. You can also try the bulletin board at the Y or your gym. Look for a distance that suits your ability and a date far enough down the road to get ready for it.
Races are almost always on weekends or holidays.
Training programs There are a host of training programs to help you prepare for races of virtually any distance. They'll tell you whether and how far to run each day of the week for a specified number of weeks leading up to the race. You can find training programs in any number of books and articles, or you can hire a running coach to design one for you. The distances build as the program progresses, then taper toward race day so you can recover in time for the event. Except for the marathon, you'll generally end up running longer than the distance you're racing toward the end of the program.