Mystery weight gain...
I trained last summer/fall for my first 1/2 marathon. Hadn't been much of a runner before. I gianed weight as well. After consulting a well trained marathoner and my aerobics instructor/personal trainer, I have learned that I may have been overeating to compensate for the feeling of, "well, I ran..."

Also, take a look at your heart rate by wearing a heart rate monitor or use the treadmill, I was over working myself and I was above the zone where I did not burn fat!