How do I get Apple to recheck my cardio fitness?

Apple watch chooses to estimate my VO2 max at weird moments, like when I am walking uphill in platform sandals. The last time it did it, I was walking with an elderly relative and not working out. How do I get it to estimate my fitness when I am working out? In the past, it always estimated me to be above average, but after the last couple times when it caught me walking in the city with platform sandals and with the elderly relative, it said I was below average for age.