As a beginning level runner, what I learned the last two weeks is:
1) Run slow enough that you keep a conversational breathing pace (or, better yet, wear a heart monitor and keep the heart rate between 140-150). Slow and steady really *does* win the race – or, at least, allow you to gradually increase distances.
2) Running too many days in a row, or increasing the distance too fast will make your knee caps ache like mine do today.
Did I mention slow and steady wins the race? :-)