It depends on the weather, especially the temperature, the distance, and your pace.  In general, you will need less than you think.  Th ideal temperature for a marathon is 50F, typically wearing shorts and a tee shirt.

Unless is it frezing or below, I am fine in running shirts, wearing an additional layer over my upper torso, perhaps also some sort of knit hat.  it is helpful if your upper garment has a full zipper, allowing ventilation.  Avoid cotton in favor of synthetics or wool.

You will feel cool at first, but after a mile or two, you will warm up and you should be fine.  That is where your full zip is handy as you fine tune to conditions.

What temps are you contemplating??

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