The Alphafly is the shoe Nike builds for the marathon. A tall ZoomX midsole, two Air Zoom units in the forefoot and a full-length carbon plate work together under an 8 mm drop, and at about 218 g it is light for how much shoe it is. Put it on when the result matters over the half or the full.
It needs pace to make sense. Run easy in it and the plate and the Air units do nothing except make the ride feel tall and odd.
It is not a trainer. The foam is fragile and the platform takes learning, so run a couple of workouts in it before race day and spend its miles carefully.