Given where we are starting from, the average American would probably be satisfied with a fairly low bar, like “not obese” or “can run a mile without stopping”. But the kind of person who writes about the topic a lot tends to be a fitness nut insisting on crazily high standards. So what makes for a reasonable middle-ground measure?
I think the US military’s standards do. They vary by branch and are changing, but here are some previous military fitness standards from the Air Force:

Here’s what the Marines expect from recruits before they show up for training:

The Army has a complex points system that varies by age and gender, but their minimum standards for a 20-year-old Male include: hex bar deadlift 150 lbs for 3 reps, 15 hand-release pushups within 2 minutes, plank for a minute 30, and a 2 mile run in 19:57 (plus their own sprint/drag/carry test in 2:28).
I like that the standards all involve a mix of strength and speed, and that they might take some work but should be achievable in a reasonable amount of time for a healthy person. I also like that they give stretch goals for the over-achievers in addition to their minimums.
What about the real over-achievers, the ones who want to be not just “in shape” but “in great shape” or “in excellent shape”? For them, there are the special forces fitness tests. Here’s the Green Berets:

The Navy SEALs naturally add a swim:

I’m in no way an authority on any of this, but for what it’s worth, you have my permission to say you’re in shape if you can meet any branch’s minimum requirements.