Let pain be your guide. If it doesn't hurt, you're okay. If it hurts, you're doing too much. Back off for a while, and then push forward again. You can easily feel the metatarsals.
Stress fractures are almost always the result of training mistakes like that.
You can get a stress fracture at the base of the fifth metatarsal, that area marked by a bump on the outside of your foot. Dancers, and basketball players even more, are susceptible to those. And jumping sports like basketball can cause a stress fracture of the navicular, the bony ridge on the top of the foot. It is the injury that basketball player Bill Walton has struggled with for years. Navicular stress fractures also are quite common in gymnasts.