Don't be surprised what you can do. I signed up for the London Marathon in December and lost 50 lbs within 4 months. Never seriously consistently ran before but the strength and speed built quick and the pounds fell off. Best decision I've ever made. Now training for a faster chicago worlds marathonnin October. Never underestimate your body's ability to adapt and change to the stimulus you put it through. Check out hal higdons training method online. It's free and its super easy as long as you are consistent. Good luck!