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I started running because of my sister. Kim and I began when we were pre-teens. I believed running was the key to making me more like her -- 5 foot 9, lean, beautiful, and highly intelligent. There was one problem: I hated running. After forcing myself to do it for a while, I was disappointed. My dreams of who I would become were dashed, and all I got from trotting around was a lower resting heart rate and bulky thighs.

Years later, much has changed -- including my relationship with running. Following her multiple-sclerosis diagnosis in 2009, Kim, who was once a long-distance runner, has retired her running shoes. She hasn't hit the pavement in more than one year. I still run. Something in my spirit won't let me quit.

Last weekend, I completed my fourth half marathon in Washington, D.C. As I toed the start line, a story began to unfold. At 7 a.m., it was barely 35 degrees outside and still dark. I was surrounded by a sea of 16,000 people -- all of whom were present for one thing: to put their bodies to the ultimate test. This morning was all about endurance.