How Do You Establish Intensity?

The intensity, this concept of VO2 max velocity or pace. How are you establishing that with each athlete? So for ultra running, it really depends on the event that the athlete is training for. If I give 24 minutes of as hard as you can go to any athlete, I know they're going to be in a VO2 range. It's just the construct of the workout kind of pigeonholes them into the right intensity.