What do you want to be fit for at 50?
A day you'd like to enjoy can tell you more about your fitness ambitions than an arbitrary birthday target.

A day worth getting ready for
Imagine a day you'd like to be able to have at fifty. You get off a train somewhere unfamiliar, walk across the city, climb the hill to the old fortress and still fancy going out for dinner. Or you spend the afternoon moving timber around a garden project. Or you join a game without spending the first ten minutes wondering whether this was a terrible mistake.
Your day might be quieter. Carrying the shopping comfortably. Getting around a place you love with less difficulty. Having something left after work for the people at home.
Now you have a different way into the fitness question. What would help you do that day?
Give the ambition a job
Wanting to look better is perfectly reasonable. Clothes fitting well can be a pleasure, and there's nothing shallow about enjoying the sight of your own progress. But appearance can leave a surprising amount unspecified.
You can picture a different waistline without knowing whether the thing you really want is the stamina for a long walk, the strength to lift something, or enough confidence in your movement to try a new activity.
Naming the activity makes the ambition more concrete. “I want to walk round Rome without organising the holiday around my tiredness” tells you something useful. So does “I'd like to join my daughter on a cycling trip” or “I want to carry my own bag when we go away.”
Those are hypothetical examples, not standards. There is no approved list of things a fifty-year-old ought to manage. Disability, illness, previous injuries, work and caring responsibilities all change the starting point. The worthwhile question is personal.
Several kinds of fit
Being good at one physical task doesn't make every other task easy. The person who can keep moving for hours may struggle with lifting. Someone strong in a familiar gym movement may feel out of their depth on uneven ground.
That isn't a defect in either person. It is a reason to notice what your chosen activity actually involves, rather than treating “fit” as one score.
A walking holiday might ask for repeated days on your feet. A particular hobby may involve awkward positions, carrying equipment or standing for long periods. Once you describe those demands, a qualified trainer or physiotherapist can help translate them into something appropriate for you if you need that support.
You don't have to create your own assessment or prove anything with a punishing first attempt. “Can I do it today?” and “Can I work towards doing it?” are different questions.
Make room for enjoyment now
Fifty can be a helpful marker. It can also become another deadline to use against yourself. There is no physical cliff edge at a birthday, and a good ambition needn't wait until then to be worth pursuing.
Try finishing this sentence: “I'd like to be able to...” Put something specific after it. Something you actually want, rather than a feat you feel you ought to admire.
Perhaps the answer is swimming on holiday, gardening for an afternoon or getting back to a sport you miss. Perhaps it's an ordinary walk which currently asks a lot of you. You can revise the ambition as your circumstances change.
The appealing bit is the day itself. The place you go, the thing you make, the people you join. Give your fitness a place in that picture.