Runner needs metabolism help

Are you sure that it is effecting your metabolism vs effecting your hunger/eating habits? According to WebMD some people gain weight and others lose weight (http://www.webmd.com/depression/ssris-myths-and-facts-about-antidepressants?page=2#1 ). It is possible that in your unmedicated state you had less hunger or desire for food, and once you were on the SSRIs you hunger "normalized". Did you track your eating when you were previously taking the SSRIs? I know from personal experience that marathon training burns an enormous amount of calories, but I also know that it is possible to out eat that burn. Trying to increase your calorie burn beyond what you already do for half and full marathon training is likely to increase your chance of injury. Which will sideline you from training and effectively will decrease your burn.

People that lose weight on SSRIs may have been using food as a way to self sooth when they were unmedicated. You should talk to your doctor if you are concerned about weight gain. There are several options within the SSRI drugs, so it might just be a matter of finding the right one.

You should also do the following:
1) Determine if you are currently at a healthy weight and how much weight can you gain and still be at a healthy weight. You might have to accept some weight gain and should know at what point to be concerned.
2) Log everything you eat and your exercise sessions, so you know if you are overeating or not.
3) Weigh yourself regularly so that you know if you are gaining too much weight.
4) Discuss all of this with your doctor(s). You want to make sure that you are in a healthy place both physically and mentally.

Good luck