I agree with the foot strike advice. A heel strike KILLS my legs. Also my stride is really small. If I want to go faster, I work on increasing my cadence rather than the length of my stride. A longer stride makes it harder to avoid landing on my heel
Shoes do make a difference too. Go to a running store if you have one near you and get someone to help figure out a good pair for you. Every foot is different! Mine have to have some cushioning in them because of my weight. I can run in "my" sneaker right out of the box without breaking them in. They work so well for me.
Treadmill is fine for now (especially if it's reeeeally cold where you live right now), but consider running outdoors on pavement and sidewalks again once you feel you can. If your races are on pavement, you should train on pavement (or sidewalks) eventually.
Also don't push yourself too hard in your training. I don't mean that in a coddling sense. I'm dead serious. The gain you get from your pain has a limit before it become a baaaaad loss. Your shorter runs are for pushing yourself. Your long runs are for no purpose other than to complete the distance. It's one thing to hurt after a run...it's another to hurt 3 days after a run.
My motivation is to run the Walt Disney World Marathon in January 2014.
Next conventions: Anime Next, Otakon
Currently working on (commission): Compa (Hyperdimension Neptunia), Rover/Fran (Animal Crossing)
Currently working on (personal): Fem!Pacha (The Emperor's New Groove)