Before baking, make sure to add corn starch too your filling - that will help thicken it. While baking, you want the filling to heat up fully - which helps to activate the corn starch. If your peach cobbler is too runny after baking, it might mean that the peaches used were super juicy, or that you didn't let it sit long enough before serving. Remember to let the cobbler cool after baking so that the filling can thicken up.