The concept of "fitness" has transformed over the past few decades, as we've developed more metrics to test, more conceptual ideas of what human fitness really is. In a biological sense, "fitness" refers to an organism's capacity to survive and reproduce. I think this is important to keep in mind, especially in this day and age. As we keep progressing towards "faster, higher, and stronger", we seem to be putting in jeopardy our survival and fertility.
Mark Sisson's recent post 11 Ways to Assess Your True Fitness Level points to many ways of thinking about fitness, all of which reflect our survival and reproductive capacity, but through different means.
Having hard metrics such as CrossFit's baseline WOD, a mile test, maximum aerobic function, max power output on the stationary bike, and Mark Rippetoe's strength standards give you true measurements which cannot be denied.
Here are Mark Rippetoe's Standards for five barbell exercises. One big take away is that we aren't really as strong as we think. The Categories listed are as follows - Cat. I: Untrained; Cat. II: Novice; Cat. III: Intermediate; Cat. IV: Advanced; Cat. V: Elite.
As an example, a 200lb man looking to bench 225lbs in the gym is barely into the Intermediate Category. A 120lb female in the Advanced Category is deadlifting 211lbs. You're never really strong "enough".
There is an old saying that goes "you can't manage what you can't measure". Thus, competing with yourself and knowing how you perform in different tests is critical to success. Your fitness pursuits should help you to become happier as a person, but don't use the excuse that you're just in it for fun as a reason to not measure progress. If you want to increase your fitness, you have to get out of a participatory mindset and set standards that are high enough that you may even fail.
These are all great ways to quantify our fitness, but the major argument is that they're measuring our functional capacity and ability to do work but they don't really measure our capacity to move. Thus, Mark Sisson throws in Gray Cook's Functional Movement Screen, and things like treading water, playing sports, walking for a long time, and getting up without using your hands.
Here's the Sitting and Rising Test. How do you score?
This is the more primitive side of our fitness - functionality and movement which is less quantifiable, but ever so important. These are things that outdoor enthusiasts and non-gym athletes will attest to which can't be emulated in the gym. In the end, you have to come up with your own definition of fitness, based on what is more important to you.
From the Movement Mindset perspective, fitness should touch on five areas:
In the end, you should ask yourself, "Does my fitness help me perform better? Does it help me to feel good and be happy?"