I've just been of the opinion that if you are going to really go after a goal, get the help of someone who has been successful in guiding others there. In the form of a coach it is literally someone who has likely learned and seen many different roadmaps to success. I go in phases myself and don't get coaching on a regular basis admittedly. But the irony is that many times when I am honed in on a goal and the end result, having someone else guide the training makes the actual process more enjoyable.
On a similar note, to me it is like a person trying to justify the cost of a good GPS watch. If you are not lucky to have to have mile markers or marked routes or to have a coach in a shadowing vehicle to help pace you, why not spend the money on a GPS. In the long run, say $300 for a Garmin, is an investment with a very high cost to benefit ratio. If it lasts even just 5 years, that is $5/month. If a person wants to really nail their goals, not sure why they'd want to leave any stone unturned. Perhaps that's just me though. Not saying that a person has to be coached 24/7 and 365, but it's worth a try and doubtful you'll end up in worst shape.