I thought about it once while riding a jeep in Death Valley National Park with my parents. The jeep had a mechanical failure. I realized that I could leave the jeep behind and walk however long it took to get to civilization (it was winter thank goodness) but they could not. To me, that is fitness.
People sometimes respond to the question with "fit for what?" Well, I'm fit for not being dependent on an automobile to save my ass in the desert, but maybe less fit for sprinting and squatting 110 pounds on my back.
Female, 5'3", 50, Starting weight: 163lbs. Current weight: 135 (more or less).
I can squat 187.5lbs, press 75lbs and deadlift 200lbs