What IanG said.
You'll lose some lean mass doing cardio, which means you can't eat as many calories on sedentary days if you want to keep losing. But it's a great calorie burner, and if you want a thinner look (as opposed to a lean muscular look) cardio is the way to go.
As for "too much" - listen to your body, as they say. If you're doing really strenuous running or uphill climbing, give your body a rest (especially at first) when your muscles are aching or tight after. That doesn't mean don't get on the treadmill, just take the difficulty down for a day, and do lots of stretching and healthy gentle stuff.
If you get to know your muscles slowly like that, you'll get a feeling for how far you can push yourself. Just don't injure yourself! Err on the side of caution, because if you hurt yourself you'll be out of commission for a while.
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