Forget longevity buzzwords like Zone 2 and VO2 max, a top personal trainer says. Focus on this simple metric.

Harley Pasternak, a celebrity trainer, has worked with clients including Rihanna. Emma McIntyre/Getty Images for Alo Yoga Tracking metrics like VO2 max and Zone 2 cardio is going mainstream via the longevity crowd. Harley Pasternak, a top trainer, said most of us don't need to track them to improve our health. Tracking your daily step count can be a good way to make sure you're moving enough. What do the terms Zone 2 and VO2 max have in common? According to the celebrity personal trainer…