Thinking back, I had a friend who ran a 29:48 10000m, and 66:xx half-marathon.
He had several goes at the marathon, and was a 100 mile a week guy at times, and couldn't ever break 2:30.
That's not to say that he may not have had the capability under the right circumstance, but if breaking 2:30 was the kind of thing you could train random 30-year-olds to do, someone with the ability of my friend would have been able to break 2:30 at the drop of the hat.
Unless you've got a pure slow-twitch guy, you are looking at a 15:30-15:40 5k type. If you go through results of races around the country the average winning time for 5k races is slower than that. So only a very, very small proportion of people with the inclination to train and compete in those races - already a massively pre-selected group - have the basic potential to break 2:30 for a marathon, and even a percentage of those are more speed-oriented guys (as I was) who aren't going to carry that level of performance up in distance.