The accuracy of your workout results depend on many factors, like: the quality of the gps receiver on your phone (varies a lot depending on the phone), the signal strength in your area (mountains, tall buildings etc), weather conditions and how you hold your phone while exercising. Make sure you have your phone in an outer pocket or preferably in your hand.
Enabling a-gps (assisted gps) that uses data connection gives more accurate results. Go to (depending on your phone this may vary) your phone settings and "location" and enable "Use networks" and "Use gps satellites". Please note that you should have a data package in order to avoid unwanted phone bills.
Also before you start a workout, always check that you have a good gps connection (the gps icon should be white). This can take up to several minutes depending on your phone and whether you're using internet connection or not.