I'd like to add one thing to the running discussion: Make sure you watch out for injuries! A minor injury could set you back a month or two, and a major injury could cause the unspeakable. I've been running 2-3 times a week since my freshman year of college, and a minor knee injury was enough to sideline me for two months (then it took me another month to return to form). Here are some tips:
1. If you haven't bought shoes in the past year, buy a new pair. Spending $60-$80 on a good pair of running shoes is well worth it, and will keep your knees fresh.
2. Avoid running on concrete if possible. Tracks are good, as are crushed-gravel paths. Grass works well too...it'll slow you down a bit, but the resistance will help in the long run.
3. Avoid taking really long strides during training, as this increases the shock on your legs.
4. Develop good running form - straight back, good breathing habits, etc.
5. If you have a pain somewhere (knees, ankles, etc), skip a day. If it just won't go away, you may want to see a doctor.
Those are the basics...there's plenty more advice all over the internet. The thing to keep in mind is that running is hell for your joints...every time you take a step, a strong shock is absorbed by your knees. Anything you can do to reduce that shock will do you very well in the long run. Hopefully this helps!