Techniques for automatic tracking of user data for exercises are disclosed. In one aspect, a method of operating a wearable device may involve determining that a user of the wearable device has started an exercise, activating the GPS receiver in response to determining that the user has started the exercise, and detecting a time at which the GPS receiver achieves an initial GPS fix of a location of the wearable device. The method may further involve logging, based on output of the one or more biometric sensors, a first set of user data during a first time interval between the start of the exercise and the detected time of the initial GPS fix, and back-filling an exercise route of the user during the first time interval based on the first set of user data.