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Your metabolism is based on how efficiently your body tissues process nutrition and so your metabolism should be great. If you're asking about causality, running more can improve your metabolism, but having a high metabolism rate does not mean you will be an excellent runner. I am an endomorph and my metabolism is naturally really bad, but after a couple years of training, I've become a pretty good runner and my metabolism has improved. I have friends though who have excellent metabolism but are terrible runners. You need stamina to run, not a good metabolism.
As a marathoner, you'll burn through lots of energy on the 26.2 mile course. Because nutritional needs and energy demands are high, runners should strive for balanced, nutritious diets in order to improve performance and endurance.