"First, the ligaments, tendons and bones will adapt more slowly to running than your cardiovascular system. By running just three or four days a week for three weeks, you can get to the point where your 2-mile or 3-mile runs start to feel easier. You may want to run faster. Resist that urge.

It takes weeks for your "structural body" to catch up to your "metabolic body." Said another way, you can build your engine—your heart and lungs—faster than your can build your chassis­—your bones, tendons and ligaments."

http://www.active.com/running/articles/running-to-lose-weight.htm