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!