Malcolm Gladwell popularized the idea that one has to do something for 10,000 hours to truly master it. I've run for more than 10,000 hours and I know a half dozen others who have as well. We as a group tend not to be swayed by the latest crazes in the sport and while usually not coaching formally, are willing to give advice to others when asked. Of this group, I'm the youngest (though one hit 10,000 hours before age 40) and the only one who's been a student of the sport - the others have largely found what worked for them early in their careers and not strayed from their usual pattern (one has simply run 10 miles per day, every day, with weekly 20 milers, for 30 years). I'm one of the less successful of the group as well, if one measures by race times; one holds multiple national records and the others have better PR's (except the one woman of the group).
One person recently pointed out an article about a coach from the 1950's. I didn't mention that the author of the article searched things I'd written for that article. I just gave the same "that's interesting" response that comes from conversations about "new" training methods or "new" diet ideas that have been around forever. There's no point in my saying I know all about it and it's meaningless - they're excited about something they've discovered and I (patronizingly and condescendingly) let them have that excitement.
The one thing that comes from all this experience is learning that you don't really know anything until what you think you know stops working. Getting a coach that's run a 2:20 marathon doesn't mean that they know how you should train to run 2:20; it just means that they were talented enough to do that once. It's the person who's on the downhill slope, whose best times are behind them, who's changed training methods repeatedly in hopes of, at first, getting the last few seconds off their PR, then of running once again close to what they've done before, then struggling to just not look horrible as they struggle to finish at all, that one wants as a coach. They've learned; they know.
There's so many people who've been running for only a few years and have had some success that are trying to get paid as coaches! They know only one way, usually, to train. Or they tried one thing before they found what worked for them. Most people will follow a standard progression: nothing works, something starts to work a little, more gets added and improvement is rapid, huge amounts of effort are put forth for a small improvement, slow decline, rapid decline, retirement. It doesn't matter, really, how they trained for most of that time, but they are all positive they know for certain the ONE TRUE WAY.
And we old-timers just shake our heads and try to get out the door for one more run.