We always did at least a 1/2 mile warmup run at a 3:30-4:00 pace and then some of the other kids would stretch. But we just had a sip of water and started the workout. If you think about it, you're just putting one foot right in front of the other. No heavy weight or strain or anything. I stretched a few times, but I was always felt more stiff when I finished the run. The only injuries I ever saw were stress related..mostly knee problems.