Arianne:
Looking at your PR's in different distances, I think you have the ability to run a low-2:30. You may say, well, those PRs were before the kids, but what we've seen is that it is realistic for a mother of several children to regain her collegiate speed when she trains and eats properly. Michelle Lowry is a good example, and you can see how she did day day by day at
http://michellel.fastrunningblog.com/ .
However, it will take training, not Runner's World/Jeff Galloway/Hal Higdon training to get ready for a race, but real training, training as a life style vs training for an event.
The time to start real training is now, and you train for several years. The key element is base mileage. You should probably start with 6 miles a day 6 days a week at a comfortable pace. Maybe 10 on Saturday. Keep adding a mile to each day as you feel your body is adapting to the training load. Work on your base mileage until you get up to 50-60 a week. Then start adding tempo runs, and make your long runs fast running the last 70% or so at a marathon pace effort.
Consistency is absolutely critical. The biggest enemy of marathon training is skipped days, which explain how the blog helps people get 30 minute marathon PRs - when you have to write about it, you hardly ever skip. Do not skip days unless you feel there are health issues such as injury or sickness. When you do not have the time, a 10 minute run is better than no run.