The active time it is recorded thanks to the motion sensor that detects only your real moving time and distance. 
When you go for a run, most of the time you’ll get the exact distance. 
Sometimes, the time displayed can be 10-20% more or less. It can take about 5 minutes until the time is updated and displayed correctly.

If the count does not seem right to you, restarting your iPhone might help fixing this problem: https://support.apple.com/en-gb/HT201559