This summer I was running 70+ mpw with a ton of motivation, spring in my step on all my runs, ability to hammer hills and crank out tempo runs, still had energy to do other things in life, and was recovering well. I was eating a lot and my weight was steady at 158-160 and I am 6 feet tall. Throughout xc my weight dropped to 148-150 and since I have been in that zone my running has gone to hell. No motivation, laboring on easy runs, tempo's feel too hard, lack of energy for class and friends, all that bad stuff. I didn't actively try to lose the weight, it just came off and I figured that was a bonus because I would run better at a lighter weight, per conventional wisdom. What gives? There are plenty of runners at 6 foot and 150 or even 140 who run very well, so why was I doing so much better when I weighed 160?