With the right program, you can train in three months provided you are running a minimum of 3-6 miles a day at the start, and, if you aren't up to speed, just add 4 weeks on to the program. I started running marathons using the 3-month plan, and from personal experience, it's not hard, just takes time and *lots* of dedication. The Chicago Marathon is usually in the fall and has a ?six hour? time limit (if memory serves me right). If someone really wanted to, they could speed walk it at that rate.
However, you are right, most runners who *really* compete usually are training year round, run more than one marathon a year as well as run other races throughout the year. And I am definitely not saying John Cusack going to run it. But he could. If he wanted to.