I have complered four marathons with times of 4:22, 4:15, 3:56, and 3:36. Only the last of those really felt like I "ran" the marathon. It required an average of 42 miles/week of training (peak week of 60 miles) for 16 weeks.
So, sure, go complete a marathon, it's a great experience. But if you want to truly "run" a marathon, for god's sake, do it right.