Ah, pity the poor maligned calorie. Rarely in history has a unit of energy been so vilified and misunderstood. You never hear people saying bad things about joules. Heck, famous authors have been named after them. Remember Joules Verne?
Good health or physical condition, especially as the result of exercise and proper nutrition.
But there's also the concept of being "fit for duty." I kind of like that: fitness as being "fit for life" as in ready to live life competently. Simply being able to live is comparatively easy in our society: eating, sleeping, breathing...muddling through each day like it's a tedious task. Being fit to live demands more than mediocrity. And that takes some effort. Is it really worth it?
Imagine you could take a pill (with no side-effects) and instantly double your strength. How would that feel? What might you do differently (other than crush your enemies, of course)? Suddenly, you could run faster, jump higher, climb, lift, throw...do all kinds of activities that were previously very difficult or even impossible. But even the simple things are affected: standing up from a chair, climbing stairs, carrying the groceries, mowing the lawn...almost everything is easier. How much would you pay for a pill like that?
You know where this is going, right? Of course. There's already a way to get those results. It doesn't happen instantly, but it also doesn't cost a fortune. It takes more time and effort than pill-popping, but if you're doing it right, that time and effort can be fun!