How fast should i run?

im 12 years old, and im homeschooled so i dont do track or anything, i just run to stay healthy and fit, and i run 3 miles 4 days a week on a hiking trail near my house, and recently i pulled my hamstring, even though i always make sure to properly warmup/cooldown and stretch, and the reason i pulled it was because i always sprint as fast as i can, so i want to slow it down a bit and i was wondering how fast should i run to stay fit and healthy and to also not injure myself, would 5-6 mph b too slow? thanks