I wasn't even running marathon distance and my upper body got pretty much slim.
But it sounds like you weren't doing weights then, until you got injured - 'Meanwhile I've started doing some full body workouts/core workout to strengthen upper body and build at least some muscles.'
Runners are by default mostly slim by the nature of their exercise so no surprises there, it's what happens when you run. If you stop running, as you did when you got injured, and then started training with weights it's no great shock that you put on muscle either.
I may be wrong, but what it sounds like you haven't been doing so far, is running and weight training at the same time? Until you are incorporating both into your fitness regime, then you cannot know that running will consume all your muscle. It won't either, so long as you train hard with the weights.