If I am on a 30-mile training ride and I want to record data for each third of a ride, how do I do this?

It appears that the lap function starts simultaneously with the real-time measurements and records lap 1 automaticially. I think all I need to do is press the lap button at the 10 and 20-mile distances to record lap data for each third of the ride. Is this correct?

I pressed the lap button as I started a ride and it showed 2 seconds for lap 1 and the remainder of the ride for lap 2. I did this thinking the lap function did not have autostart. I meant to record other segments of the ride as laps but forgot since this was my first time using the computer.