I began running in September of 1999. Although I had never run before I was in decent shape and could comfortably run around 30 minutes. If you and your mother are not at this point I would suggest that you alternate runnning and walking gradually decreasing the amount of walking until you can run 30 minutes.
From this point I gradually would increase the time I ran until I reached an hour, which was a huge milestone. It took me around 2 months to work up from being able to run 30 minutes to being able to run 1 hour comfortably. It was at this point I signed up for a marathon, which was around six (6) months away. I downloaded one of the many training schedules available on the web and bought a book called Four Months to a Four Hour Marathon, which I found useful. I ran my first marathon in May 2000 in Cincinnati in 4:10. Since then I have run Chicago and Disney, with my PR being the 4:02 I ran in Chicago last year. I hope to break four hours next week in Cincinnati.
I would say that from my experience it took me around eight (8) months to prepare.