I've never been one of those "what gets measured gets managed" kinda guys, but as a 40 year-old I do find this from my new Garmin watch quite motivating...
"Fitness Age is a metric tied to your VO2 Max fitness level. While your VO2 Max estimate describes your current fitness level, it’s not always easy to know what it means. Fitness Age reinterprets your VO2 Max score in terms of age to make it more relatable. Regularly engaging in the right types of physical activity will help you boost your VO2 Max estimate score and will reduce your Fitness Age."
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