How can I get fit without losing weight?

I'm 13 years old (nearly 14), I'm 5ft4" and weigh 8 stone. For about the past 5/6 months I've been suffering from knee pain. I went to see my GP and he said to keep active as it was probably only 'growing pains'. So, as I'm happy with my weight and don't want to lose any, how... show more I'm 13 years old (nearly 14), I'm 5ft4" and weigh 8 stone.

For about the past 5/6 months I've been suffering from knee pain. I went to see my GP and he said to keep active as it was probably only 'growing pains'.

So, as I'm happy with my weight and don't want to lose any, how can I keep fit without losing any of it? I'll be using a treadmill if that makes any difference whatsoever.

Any help will be much appreciated!
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