What's a good real-world (i.e. not something like VO2max or in a gym) measure of general fitness? Something that most adults would be able to do, and measuring performance, not achievement. (So not anything like "if you can run a marathon...")
@levisan if you can keep up with a 7 yr old after 4 bags of skittles and 5 Mountain Dews.
@levisan If you can find it, Sally Edwards' The Heart Rate Monitor Book does a pretty good job of covering the basics.
I've been cycling for decades using HRMs, but I'm not really an expert, nor an elite athlete. My usual checks are resting heart rate and how quickly HR recovers after an interval day or other short intense session (including weight room days). Key is to measure and log over time, not generic formulas. Garmin or Apple fitness watches make it so much easier, but get expensive.