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Be prepared for all kinds of weather you could possibly encounter when selecting running clothes for a marathon. Rain, heat, wind, cold. Unless you are running in a location where the climate factors are known (it's not going to be cold in Miami in August), it's best to have gear ready for all occasions.
Hat: I like to always bring a light weight hat in case of rain or just to keep the sun off of my face and head. Your hat should have a visor and consist of light-weight breathable material (not a hat that retains water and becomes heavy). White is better for a sunny day, otherwise color should not matter.
Shirt: NO COTTON! Wear something that wicks moisture away from that body and that you feel comfortable in. The fit should feel a little loose, preferably but not necessarily sleeveless or a tank top. A common problem with marathons is early morning starts, when the temperature starts cool and begins to warm up. I make it a practice to bring a "throw away" shirt, and wear it until I no longer need it. If it's going to be cold, you are much better off wearing 3 layers of light weight gear than 1 heavy top.
I would only consider tights if the temperatures are forecast to remain 45 degrees or below. I can't tell you how many times I have seen women in particular wearing tights on a day when temperatures climb into the 50s or even 60s. Trust me, your legs are not going to be cold.
Shorts: No rub spots on the inner thighs! Buy shorts made with light-weight nylon, built or other high tech fiber that won't absorb moisture.
Sun Prevention: Sunglasses and sunscreen (at least 30 SPF).
Gloves: If temperatures warrant, don't forget some light-weight gloves. I prefer something inexpensive in case I want to ditch them along the way.
Plastic Garbage Bags: To wear in case you have cool rain prior to starting.
(c) Dave Elger 2008 All rights reserved