I was stuck in a similar situation for months, and put on quite a bit of weight as a result. I really liked running (I still do) and felt like nothing else could come close. However, you can always find another sport. I got on my bike a lot more, and when I found that to be not ideal, I started to go swimming. I still don't like swimming, but it lets me experience much of the same post-workout mental boost that running used to give me. After a few months, I am fortunately back to running now, but I still try to vary what I do, getting at least one bike and two swimming sessions in per week. Cross-training not only helps you stay fit while you're injured, it also helps prevent injuries in the future. I am sure you can think of *something* you can do that doesn't hurt your knees - and if you can think of something, then by all means, go and do it. You will feel so much better afterwards!
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