I can actually speak from experience on this point. Most people (including me) begin running marathons once they are done racing on the track. The others are people that never ran competitively when they were younger and simply get into marathoning as something to do for fitness.
When I ran competitively, I ran under 14:10 for 5k. The last time I ran a 5k over 17:00 I was probably in the 10th grade, but the last time I ran a 5k was when I was 30.
Over the past few years I've jumped into a few marathons with varying degrees of disappointment. I only trained for one of them, and that was the only one in which I broke 3:00.
I remember regularly doing 20 mile long runs under 6:15 pace. I have no doubt that back then I could have somewhere in the area of 2:35 (maybe faster) with no additional training. I also suspect that most college athletes that can run under 17:00 for 5k would be able to run under 3:00 for a marathon without any additional training.
However, experience has also taught me that no matter how fast I was when I was young and fit, simply throwing on some shoes (without any preparation) and running a marathon under 3:00 is considerably more difficult than it used to seem. It is also extremely painful, and, as I reflect on my experiences, a terrible idea.