It's rather easy (as many of you have noted) for collegiate distance runners to gain weight quickly after graduation. Training for competitive distance running induces a catabolic environment (high cortisol, yada..) which will naturally prevent weight gain. Remove the many miles and intense training while likely maintaining caloric intake will definately lead to rapid weight gain. I've have no scientific evidence, but it is logical to suspect that a distance athlete coming off training may experience an accelerated protein syntheis rate that would enhance the muscle accretion. To be simplistic, the body has a set point body weight and under normal homeostatic conditions (not running 100 mi/wk), the desire for your body to get to that weight is rather strong.
To attempt to answer your original question, 15 lbs of any weight, especially in the upper body will likely do nothing to improve your marathon performance. Proper long runs will easily take care of the excess fat and as you sharpen closer to the marahon, i would suspect that you will find yourself back at your collegiate race weight.
Good Luck.