In the PB fitness e-book Mark suggests sprinting on an incline first to help prevent injury, it worked pretty well for me. Also I find that swimming is a great way to either do "sprints" or some fun low level activity without injury. Most importantly if anything starts to hurt or feel off just stop, even if you feel like you haven't put in a good workout. A half-finished workout is better than a painful full length workout with an injury. After injuring myself multiple times from pushing it to hard, I learned. Start slow and you will see improvements! Good Luck!