I ran my first marathon about 5 years ago. At the time I was running 3 miles about 4 times a week at 10min/mile pace. It's totally feasible if you have the patience to train for it. And it does great things for the lungs! Just make sure (especially when running outside in the summer) that you drink enough and take salt tablets to replenish your system. Hal Higdon has some great novice training programs to follow. Good luck!