From reading the article, the example as stands will give you a GEOGRAPHY
point object (although not sure if your inputs are valid, it seems it require a street address). Anyway, call it twice then to determine the distance between the points returned use something like
Point1.STDistance(Point2)
Here's the BOL reference https://msdn.microsoft.com/en-us/library/bb933808.aspx
Alternatively, see this question for a Bing Routes API answer. How to calculate the driving distance between two points?
If you still need to geocode the approx centre of a zipcode, you might be better off importing the data yourself. I did this recently for the Australian GNAF data (https://www.rednotebluenote.com/2016/04/importing-psma-geocoded-national-address-file-g-naf-to-sql-server/ ) which you might find useful background reading. For US I believe http://census.gov/ makes something similar available.
Hope this is of use.