Greg - I don't know anything about you, although I get that you are one of those quasi-celebrity posters on this forum. There's you, jamin, and a few other people. So you were semi-decent at age 19 and are trying to credit yourself with pretty much having demonstrated 2:45-2:48 ability in the marathon without having actually run one?
No way, my friend. Maybe you could have, and maybe you couldn't have. I'm not trying to claim any mystical knowledge of the marathon (my PB is 2:32 - somewhere in the not-terrible to unremarkable range, and right in line with my other PBs), but some people can be pretty good at the mile, 5k, 10k, etc, etc, but it just doesn't translate to the marathon. Even some elite distance runners never could get it right for a marathon (Ritzenheim, Tadesse, others). Simply put, until you find out whether you are someone who tends to fall apart sometime after 16 miles or so, you can't claim anything. Anybody can look at a conversion chart and see that a 15:00 5K equates to a 2:28 marathon, or something like that, but that doesn't mean much - if you take 100 15:00 5k runners and have them run a marathon, some will fail miserably and end up a DNF or over 3 hours.
So while it is your right to not want to run a marathon, I'd recommend giving up speculating on what you "could have run" for one. A large part of the appeal of a marathon - and why it's good that it ended up being 26.2 miles instead of, say, 20 miles - is that it taps into a whole different aspect of endurance running. Actually committing to a marathon - well, I don't want to say it takes guts, because that would mean patting myself on the back, and I'm certainly nothing remarkable - but it does seem to show a certain seriousness of purpose, let's say - because you expose yourself and run the risk of failing. By contrast, that doesn't happen in a 10K. In a 10k, at worst you have a sub-par race, and you can just try again next weekend.