One important thing is to run heavy/light, where those words refer to the stress on your body not to your effort in doing the running. A light day might be a rest day. Or, it could be a cross training day of light swimming, cycling, or walking. It could be a day of light running, say doing half the distance and maybe a bit slower. As irunnerblog said, your body needs days to recover from your running. Another important thing is to not add more than about 10% (or less) per week to your distance.
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