Does kondition always refer to fitness in a positive sense? I read this as "My condition / health is not what it was.
Could this also be said by someone who once was seriously ill but is now improved, recovered, or even glowing with health?
No, it doesn't say anything about health at all, really. It's essentially fitness as a measure of how well you're able to keep at endurance or cardio exercise. If you have bad kondition, it means you're an unfit runner, or cyclist, etc.