Hi Carol, you haven't killed your starter (which is definitely a mature starter) by feeding it extra flour, although it may rise a bit slower and puff up more like a dough. Essentially you've temporarily converted your starter to a "stiff" starter, although to make this transition complete it would take several more feedings at room temperature for your starter to fully adapt to its new feeding routine and consistency. Note that our stiff starter recipe also offers instructions on how to convert back to a liquid starter in the Tips from our Bakers section: Want to turn your stiff starter back into a liquid starter? Rip the stiff starter into small pieces and mix it with lukewarm water. The ratio is 3 parts starter to 1 part water, by either weight or volume: e.g., 3/4 cup (6 ounces) stiff starter added to 1/4 cup (2 ounces) water. The easiest way to mix the stiff starter and water is at low speed in a stand mixer, using the beater attachment. 

I hope this helps! Let us know how it goes. 

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