Definitely look at shortening your stride - if your foot is landing ahead of you it actually uses more energy and has a higher impact on your body, as you are essentially "braking" and then re-starting your motion.

When I first started running I was going way too fast to build up any distance - my first race was an 8K that I ran at an 8:20 pace (probably only endured for 5 miles because it was 20 degrees outside). Now (meaning before I hurt my back) I run a 10K at essentially a 9-minute pace and a marathon at around 10:15/mile.