I just ran my first marathon in mid-June. I'd say I clocked in about 15-17 hours a week for 18 weeks for the training. On top of training, I work 40+ hours a week, was studying for the DAT for 3 of those months, traveled, participated in other sports such as skiing, and spent the evenings at home with my doggie Scooter. 😉 The biggest time constraint was saving time in my busy schedule to make 3 healthy meals. You will find that you can't train without the proper foods. Your body will quit on you. I can see myself continue running (running to the extreme, as my friends put it) in dental school. It's all about good time management and giving up Friday and Saturday nights for something that is important to you.
On a related note, I have a friend at Yale Med (2nd year) who has competed in 3 marathons while a medical student and this summer is doing a triathalon.