It takes about 5 mi(approx 8km) of traveling to fill a distance bar.
By activating the Accelerator using the Boost button below the meter, the distance bar will fill up with only half the usual distance required.
If the bars don't fill up faster even after trying the above measures
It is possible that location measurements may have temporarily stopped or were not taken while the app was not in use.
If the problem persists, try the procedures described in [I am traveling but the distance bar is not filling].