I don't know how this is in Swedish, but in German "having a good 'Kondition'" means having good stamina. Is this similar in Swedish or does it just mean "being fit" in general?
That's right, it refers to e.g. your running endurance, not to your muscle strength.
Thank you! It wasn't explained very well.
Why not 'i am in a good condition'
Condition/Kondition is a false friend. Condition in English translates to "skick", referring to a general state of something. Kondition in Swedish refers to physical fitness mostly.
In the uk fit is a collequilism (idk how its spelt... ) for attractive can this be used in the same way
No, not at all.
Can this be used when you've been sick and you feel better now? Or is it only about looks?
Neither, actually. It refers to endurance fitness, as in having good kondition meaning you don't get out of breath easily. And it should really be god kondition, as that's somewhat more idiomatic in Swedish.