Is 108 too low a number?

I'm 5'6 and 15 almost 16. I run Cross Country, and as lovely as it is to me I'm gaining weight. Muscle or not, I want to be very light as it makes running easier. If I restrict some food intake and keep up a minimum of running 4 miles daily, would 108 pounds still be too low for me to achieve? Would it be unhealthy?

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