How accurate Endomondo really is depends on various factors like:
How good and constant the GPS lock is. High end phones like Galaxy S4, Note 3, HTC One or iPhone 5 are best for Endomondo GPS.
Where the phone is on your person throughout the workout.
Weather or not it has access to data throughout your workout. Some spots Sprint’s data doesn’t work or there is no good tower cover so make sure you are with a carrier that has solid LTE or 4G data reliability like Verizon or AT&T. Just make sure you set everything right before your workout so you don’t end up having issues later.