The German adjective "fit" is a relatively recent loanword from English. However, whereas this word can mean both “suitable” and “in good physical condition" in English, only the second definition is applicable in German. 

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The German adjective fit is used not only to describe physical fitness, but also relates to mental health, energy, and absence of illness. Here are some examples with both of these connotations:

So, ja, das ist ein Trainings-Trial für mich, um in der deutschen Meisterschaft wieder richtig fit zu sein.

So, yes, this is a training trial for me, to be really back in shape for the German championship.

Captions 39-40, Trial-Meisterschaft: in Bensheim

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Hey, ich bin Erik und ich mache euch jetzt fit!

Hey, I'm Erik and now, I'm going to get you into shape!

Caption 1, Workout mit Erik: Übungen für Arme, Beine, Po, Rücken

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Topfit und ohne Sorgen.

Fit as a fiddle and without worries.

Caption 2, Deine Freunde: Du bist aber groß geworden

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Ich gehe früh ins Bett, damit ich morgen fit bin.

I'm going to bed early so that I'll be in good condition tomorrow.

Caption 23, Deutsch mit Eylin: Konjunktionen

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Werde auch du zum Helden! Bleibe zuhause und halte dich fit.

Become a hero too! Stay at home and stay healthy.

Caption 36, Die Corona-Krise: Zusammen bewegen gegen Corona

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If you're wondering how to say that something is suitable—as in "The film is suitable for children" or "A meal fit for a king"—there are two adjectives that work well. In last week's lesson, we looked at the verb passen, which is related to the adjective passend.

Ich sollte sicherstellen, dass ich etwas Passendes für das Geschäftsumfeld finde.

I should make sure that I find something fitting for the business environment.

Captions 72-73, Berufsleben: das Vorstellungsgespräch

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And then there is also geeignet, which is derived from the verb sich eignen. This has the meaning of not only being compatible, but also qualified and appropriate. 

Kartoffeln, Würstchen, Sauerkraut und Schwarzwälder Kirschtorte sind natürlich die deutschen Klassiker, aber sicher nicht für jeden Tag geeignet.

Potatoes, sausages, sauerkraut, and Black Forest cake are of course the German classics, but certainly not suitable for every day.

Captions 16-18, Deutsch mit Eylin: Rund ums Kochen

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Further Learning
You'll find more examples of these adjectives on Yabla German.