I actually did a marathon when I was 16. I ran about 3:15 or so, then ran 3:00 the next year. while it was really fun, I had to compromise between training for track or training for the marathon. The first year I pretty much just did normal track training, then did a few 16-19mi long runs after the track season was over, then ran the marathon. The second time, I did some long runs during the track season, but it destroyed a lot of opportunities for me to race well. I was probably in about 10:00 2mi shape but I would go out in 5:00 then blow up and run 10:30. It is also hard to get a good summer (or winter in your case) of training after running a marathon, since you need a week or two before you'll really be able to get out and train again. I could hardly get down the stairs the first few days my sophomore year.
Anyhow in short I don't recommend trying to do both. Leave the marathon until after college. If you are a sophomore able to run 16:30, college running is definitely a good idea.
As for biking to school, it all depends how far. I biked 2mi to school and back every day and I was fine. It's probably better for you than sitting on your butt in a car. Just don't try to time-trial it...take it easy.