If your peanut butter frosting is too runny, it's possible you used a natural type of peanut butter where the oil separates from the peanut butter. Next time make sure to use a regular peanut butter, not a natural variety. For now, add 1/4 cup of extra powdered sugar and omit the milk. If it's still too runny, add a bit more powdered sugar until you get the desired consistency you're looking for.