Weight is absolutely critical whether you get there via diet or exercise. Try running with a 40 pound weighted vest sometime. I've read somewhere that an ideal weight is 2X your height in inches. So, at 5'7 you should weigh about 134 (2X 67 = 134). Body fat is very important. If you're serious about running you should be down around 5-6% body fat. you only need a bathroom scale to be 97% accurate versus the Decla "gold standard" which will cost you mega bucks to be done at a hospital/clinic. The bathroom scales work through electrical impedance.