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Training for a marathon involves running for hours at a time, and if you have an ill-fitting sports bra, you will know it by the chafe marks you'll see (and feel) after the run. If you don't already have a favorite sports bra, try on a few styles and find one that feels comfortable in the store. If it's not comfortable when you put it on, it probably won't feel comfortable after 15 miles of running. But it should feel snug; you don't want to be bouncing. Read the labels and make sure that the bra advertises maximum support for the cup sizes. If your local store doesn't have a spectrum of sizes and styles that appeal to you, user-friendly online resources include championcatalog.com and roadrunnersports.com.
Ideally, don't wear a brand new bra on the day of a race or a long run. Test it out on short runs first, and if you're not happy with it, either exchange it, or if it is comfortable but not supportive enough for running, keep it to use for the gym or yoga or another low-impact activity.