This is the perspective from someone who has trained themselves to two OT's.
No one knows how to train themselves from day 1. This should be obvious. I learned a lot from my high school and college coaches and continued my running education from other athletes and coaches post collegiately. I absolutely would not have been able to effectively coach myself post collegiately without the experience gained from my prior coaches over almost a decade.
I think even now I would benefit from the right coach. Even if it was just at times getting a second perspective or telling me to slow down when I'm pushing to hard. However with my mix of running and work I haven't found a coaching situation I would want and would rather just consult with those I trust when I feel I need it. I also feel the gain to me would not be worth it. If I was a 2:10 guy and the right coach could make a 1 or 2 minute difference and potentially be the difference in making an Olympic team I would have a different perspective. But just moving up a notch on the sub elite ladder I have a different perspective.