When you are ready, there is an interesting article from Runners World magazine last year about a marathon training program with only 3 days of running per week. It is on their website under training plans. The idea is to cross train on other days at a high intensity and the running days are higher quality workouts. It is nice to be able to work biking and swimming, or whatever, into your training, but still focus on the running.
You still need a solid base before attempting a marathon, but I do think the cross training helps with injuries.