Garmin coach Stride repeats

Hello

I'm new to Garmin and would be very grateful for assistance using the coaching app. My next workout is Stride Repeats. After the warm up there is an instruction to press the lap key. The rest of the workout is sprint for 20 seconds and rest for 45 seconds. Will the app measure the seconds for each or do I need to press the lap key after 20 seconds and then again after 45 seconds. 

Thanks

Liz

hope this makes sense?