Alan, I think you’ll get different answers from different coaches on this question. I’m fairly conservative, and I like to use the “maximum effective dose” principle of training. This represents the training level above which no improvement in efficacy is obtained. Of course, that efficacy is relative to the distance. The maximum effective dose for a 5K is probably 1.5 hours, but the maximum effective dose for a marathon could be up to 3 hours.
I can’t claim that our Level 3 HM plan is as good as it gets, it’s not the maximum effective dose, it’s written for the masses. We could probably put together a Level 4 or 5 HM plan, or a custom plan that would out-perform the Level 3, but I am saying the Level 3 HM plan is pretty close to as good as you need for non-pro athletes.
However, you’ll almost certainly perform a HM better using a marathon plan than the HM plan NOT because the Marathon plan represents the maximum effective dose, but because the maximum effective dose is somewhere slightly north of the HM Level 3 plan, but well south of the Marathon Level 2 plan.
Sheesh, long answer. OK, here’s the short version: It’s more than necessary, but you’ll probably perform very well if you ensure that recover appropriately and spend sufficient practicing time at HM pace.