Marathon training is one of the few times you don't want to facilitate the hormetic response to exercise. You want antioxidants and immunostimulants.

Vitamin C, Echinacea, Anaycyclus pyrethrum, NAC, LCLT.

Pre-marathon, a dietary protocol of glycogen depletion, followed by low/no carbs for 2-3 days, followed by carb-loading should create a strong glycogen supercompensation response. Since glycogen depletion is tied with failure in marathons, you want as much as possible going in.